gdb: work around negative DW_AT_data_member_location GCC 11 bug

gdb: work around negative DW_AT_data_member_location GCC 11 bug

g++ 11.1.0 has a bug where it will emit a negative
DW_AT_data_member_location in some cases:

    $ cat test.cpp
    #include <memory>

    int
    main()
    {
      std::unique_ptr<int> ptr;
    }
    $ g++ -g test.cpp
    $ llvm-dwarfdump -F a.out
    ...
    0x00000964:       DW_TAG_member
                        DW_AT_name [DW_FORM_strp]   ("_M_head_impl")
                        DW_AT_decl_file [DW_FORM_data1]     ("/usr/include/c++/11.1.0/tuple")
                        DW_AT_decl_line [DW_FORM_data1]     (125)
                        DW_AT_decl_column [DW_FORM_data1]   (0x27)
                        DW_AT_type [DW_FORM_ref4]   (0x0000067a "default_delete<int>")
                        DW_AT_data_member_location [DW_FORM_sdata]  (-1)
    ...

This leads to a GDB crash (when built with ASan, otherwise probably
garbage results), since it tries to read just before (to the left, in
ASan speak) of the value's buffer:

    ==888645==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6020000c52af at pc 0x7f711b239f4b bp 0x7fff356bd470 sp 0x7fff356bcc18
    READ of size 1 at 0x6020000c52af thread T0
        #0 0x7f711b239f4a in __interceptor_memcpy /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:827
        #1 0x555c4977efa1 in value_contents_copy_raw /home/simark/src/binutils-gdb/gdb/value.c:1347
        #2 0x555c497909cd in value_primitive_field(value*, long, int, type*) /home/simark/src/binutils-gdb/gdb/value.c:3126
        #3 0x555c478f2eaa in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:333
        #4 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #5 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #6 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #7 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #8 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #9 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #10 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #11 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #12 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #13 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #14 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #15 0x555c478f2fcb in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:335
        #16 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #17 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #18 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #19 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #20 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #21 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #22 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #23 0x555c478f2fcb in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:335
        #24 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #25 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #26 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #27 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #28 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #29 0x555c4760f04c in c_value_print(value*, ui_file*, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:587
        #30 0x555c483ff954 in language_defn::value_print(value*, ui_file*, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:614
        #31 0x555c49759f61 in value_print(value*, ui_file*, value_print_options const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1189
        #32 0x555c48950f70 in print_formatted /home/simark/src/binutils-gdb/gdb/printcmd.c:337
        #33 0x555c48958eda in print_value(value*, value_print_options const&) /home/simark/src/binutils-gdb/gdb/printcmd.c:1258
        #34 0x555c48959891 in print_command_1 /home/simark/src/binutils-gdb/gdb/printcmd.c:1367
        #35 0x555c4895a3df in print_command /home/simark/src/binutils-gdb/gdb/printcmd.c:1458
        #36 0x555c4767f974 in do_simple_func /home/simark/src/binutils-gdb/gdb/cli/cli-decode.c:97
        #37 0x555c47692e25 in cmd_func(cmd_list_element*, char const*, int) /home/simark/src/binutils-gdb/gdb/cli/cli-decode.c:2475
        #38 0x555c4936107e in execute_command(char const*, int) /home/simark/src/binutils-gdb/gdb/top.c:670
        #39 0x555c485f1bff in catch_command_errors /home/simark/src/binutils-gdb/gdb/main.c:523
        #40 0x555c485f249c in execute_cmdargs /home/simark/src/binutils-gdb/gdb/main.c:618
        #41 0x555c485f6677 in captured_main_1 /home/simark/src/binutils-gdb/gdb/main.c:1317
        #42 0x555c485f6c83 in captured_main /home/simark/src/binutils-gdb/gdb/main.c:1338
        #43 0x555c485f6d65 in gdb_main(captured_main_args*) /home/simark/src/binutils-gdb/gdb/main.c:1363
        #44 0x555c46e41ba8 in main /home/simark/src/binutils-gdb/gdb/gdb.c:32
        #45 0x7f71198bcb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)
        #46 0x555c46e4197d in _start (/home/simark/build/binutils-gdb-one-target/gdb/gdb+0x77f197d)

    0x6020000c52af is located 1 bytes to the left of 8-byte region [0x6020000c52b0,0x6020000c52b8)
    allocated by thread T0 here:
        #0 0x7f711b2b7459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
        #1 0x555c470acdc9 in xcalloc /home/simark/src/binutils-gdb/gdb/alloc.c:100
        #2 0x555c49b775cd in xzalloc(unsigned long) /home/simark/src/binutils-gdb/gdbsupport/common-utils.cc:29
        #3 0x555c4977bdeb in allocate_value_contents /home/simark/src/binutils-gdb/gdb/value.c:1029
        #4 0x555c4977be25 in allocate_value(type*) /home/simark/src/binutils-gdb/gdb/value.c:1040
        #5 0x555c4979030d in value_primitive_field(value*, long, int, type*) /home/simark/src/binutils-gdb/gdb/value.c:3092
        #6 0x555c478f6280 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:501
        #7 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #8 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #9 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #10 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #11 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #12 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #13 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #14 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #15 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #16 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #17 0x555c478f2fcb in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:335
        #18 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #19 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #20 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #21 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #22 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #23 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #24 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #25 0x555c478f2fcb in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:335
        #26 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #27 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #28 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #29 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048

Since there are some binaries with this in the wild, I think it would be
useful for GDB to work around this.  I did the obvious simple thing, if
the DW_AT_data_member_location's value is -1, replace it with 0.  I
added a producer check to only apply this fixup for GCC 11.  The idea is
that if some other compiler ever uses a DW_AT_data_member_location value
of -1 by mistake, we don't know (before analyzing the bug at least) if
they did mean 0 or some other value.  So I wouldn't want to apply the
fixup in that case.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28063
Change-Id: Ieef3459b0b9bbce8bdad838ba83b4b64e7269d42