Test case reproducing PR28030 bug

Test case reproducing PR28030 bug

The original reproducer for PR28030 required use of a specific
compiler version - gcc-c++-11.1.1-3.fc34 is mentioned in the PR,
though it seems probable that other gcc versions might also be able to
reproduce the bug as well.  This commit introduces a test case which,
using the DWARF assembler, provides a reproducer which is independent
of the compiler version.  (Well, it'll work with whatever compilers
the DWARF assembler works with.)

To the best of my knowledge, it's also the first test case which uses
the DWARF assembler to provide debug info for a shared object.  That
being the case, I provided more than the usual commentary which should
allow this case to be used as a template when a combo shared
library / DWARF assembler test case is required in the future.

I provide some details regarding the bug in a comment near the
beginning of locexpr-dml.exp.

This problem was difficult to reproduce; I found myself constantly
referring to the backtrace while trying to figure out what (else) I
might be missing while trying to create a reproducer.  Below is a
partial backtrace which I include for posterity.

 #0  internal_error (
    file=0xc50110 "/ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/gdbtypes.c", line=5575,
    fmt=0xc520c0 "Unexpected type field location kind: %d")
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdbsupport/errors.cc:51
 #1  0x00000000006ef0c5 in copy_type_recursive (objfile=0x1635930,
    type=0x274c260, copied_types=0x30bb290)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/gdbtypes.c:5575
 #2  0x00000000006ef382 in copy_type_recursive (objfile=0x1635930,
    type=0x274ca10, copied_types=0x30bb290)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/gdbtypes.c:5602
 #3  0x0000000000a7409a in preserve_one_value (value=0x24269f0,
    objfile=0x1635930, copied_types=0x30bb290)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/value.c:2529
 #4  0x000000000072012a in gdbscm_preserve_values (
    extlang=0xc55720 <extension_language_guile>, objfile=0x1635930,
    copied_types=0x30bb290)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/guile/scm-value.c:94
 #5  0x00000000006a3f82 in preserve_ext_lang_values (objfile=0x1635930,
    copied_types=0x30bb290)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/extension.c:568
 #6  0x0000000000a7428d in preserve_values (objfile=0x1635930)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/value.c:2579
 #7  0x000000000082d514 in objfile::~objfile (this=0x1635930,
    __in_chrg=<optimized out>)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/objfiles.c:549
 #8  0x0000000000831cc8 in std::_Sp_counted_ptr<objfile*, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=0x1654580)
    at /usr/include/c++/11/bits/shared_ptr_base.h:348
 #9  0x00000000004e6617 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x1654580) at /usr/include/c++/11/bits/shared_ptr_base.h:168
 #10 0x00000000004e1d2f in std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=0x190bb88, __in_chrg=<optimized out>)
    at /usr/include/c++/11/bits/shared_ptr_base.h:705
 #11 0x000000000082feee in std::__shared_ptr<objfile, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=0x190bb80, __in_chrg=<optimized out>)
    at /usr/include/c++/11/bits/shared_ptr_base.h:1154
 #12 0x000000000082ff0a in std::shared_ptr<objfile>::~shared_ptr (
    this=0x190bb80, __in_chrg=<optimized out>)
    at /usr/include/c++/11/bits/shared_ptr.h:122
 #13 0x000000000085ed7e in __gnu_cxx::new_allocator<std::_List_node<std::shared_ptr<objfile> > >::destroy<std::shared_ptr<objfile> > (this=0x114bc00,
    __p=0x190bb80) at /usr/include/c++/11/ext/new_allocator.h:168
 #14 0x000000000085e88d in std::allocator_traits<std::allocator<std::_List_node<std::shared_ptr<objfile> > > >::destroy<std::shared_ptr<objfile> > (__a=...,
    __p=0x190bb80) at /usr/include/c++/11/bits/alloc_traits.h:531
 #15 0x000000000085e50c in std::__cxx11::list<std::shared_ptr<objfile>, std::allocator<std::shared_ptr<objfile> > >::_M_erase (this=0x114bc00, __position=
  std::shared_ptr<objfile> (expired, weak count 1) = {get() = 0x1635930})
    at /usr/include/c++/11/bits/stl_list.h:1925
 #16 0x000000000085df0e in std::__cxx11::list<std::shared_ptr<objfile>, std::allocator<std::shared_ptr<objfile> > >::erase (this=0x114bc00, __position=
  std::shared_ptr<objfile> (expired, weak count 1) = {get() = 0x1635930})
    at /usr/include/c++/11/bits/list.tcc:158
 #17 0x000000000085c748 in program_space::remove_objfile (this=0x114bbc0,
    objfile=0x1635930)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/progspace.c:210
 #18 0x000000000082d3ae in objfile::unlink (this=0x1635930)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/objfiles.c:487
 #19 0x000000000082e68c in objfile_purge_solibs ()
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/objfiles.c:875
 #20 0x000000000092dd37 in no_shared_libraries (ignored=0x0, from_tty=1)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/solib.c:1236
 #21 0x00000000009a37fe in target_pre_inferior (from_tty=1)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/target.c:2496
 #22 0x00000000007454d6 in run_command_1 (args=0x0, from_tty=1,
    run_how=RUN_NORMAL)
    at /ironwood1/sourceware-git/f34-pr28030/bld/../../worktree-pr28030/gdb/infcmd.c:437

I'll note a few points regarding this backtrace:

Frame #1 is where the internal error occurs.  It's caused by an
unhandled case for FIELD_LOC_KIND_DWARF_BLOCK.  The fix for this bug
adds support for this case.

Frame #22 - it's a partial backtrace - shows that GDB is attempting to
(re)run the program.  You can see the exact command sequence that was
used for reproducing this problem in the PR (at
https://sourceware.org/bugzilla/show_bug.cgi?id=28030), but in a
nutshell, after starting the program and advancing to the appropriate
source line, GDB was asked to step into libstdc++; a "finish" command
was issued, returning a value.  The fact that a value was returned is
very important.  GDB was then used to step back into libstdc++.  A
breakpoint was set on a source line in the library after which a "run"
command was issued.

Frame #19 shows a call to objfile_purge_solibs.  It's aptly named.

Frame #7 is a call to the destructor for one of the objfile solibs; it
turned out to be the one for libstdc++.

Frames #6 thru #3 show various value preservation frames.  If you look
at preserve_values() in gdb/value.c, the value history is preserved
first, followed by internal variables, followed by values for the
extension languages (python and guile).