Fix "info break" + "catch catch" + -static-{libstdc++,libgcc}

Fix "info break" + "catch catch" + -static-{libstdc++,libgcc}

If you debug current GDB, set a "catch catch/throw/rethrow"
catchpoint, and then do "info breakpoints", the top GDB hits an
internal error:

 (top-gdb) catch catch
 Catchpoint 1 (catch)
 (top-gdb) info breakpoints
 Num     Type           Disp Enb Address            What
 1       breakpoint     keep y   src/gdb/breakpoint.c:6040: internal-error: void print_one_breakpoint_location(breakpoint*, bp_location*, int, bp_location**, int): Assertion `b->loc == NULL || b->loc->next == NULL' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n)

The assertion in question is asserting that a breakpoint with a
print_one method only has one location, and it fails because this
catchpoint ends up with two locations.

Internally, "catch catch" sets a breakpoint at __cxa_begin_catch.  If
we do that manually, we see the locations:

  (top-gdb) b -qualified __cxa_begin_catch
  Breakpoint 2 at 0xb122b0 (2 locations)
  (top-gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  2       breakpoint     keep y   <MULTIPLE>
  2.1                         y   0x0000000000b122b0 <__cxa_begin_catch>
  2.2                         y   0x00007ffff2f4ddb0 in __cxxabiv1::__cxa_begin_catch(void*) at ../../../../libstdc++-v3/libsupc++/eh_catch.cc:41

Note that I had used -qualified.  It seems strange that we get a
location for a namespaced symbol, but that happens because the minimal
symbol for that address is indeed called __cxa_begin_catch.

The real issue is that gdb is linked with
-static-libgcc/-static-libstdc++.  And then, it _also_ ends up with
shared libstc++ loaded:

  (top-gdb) info sharedlibrary stdc++
  From                To                  Syms Read   Shared Object Library
  0x00007ffff2f4b380  0x00007ffff2ffc018  Yes         /lib64/libstdc++.so.6

Location 2.2 is set within libstdc++.so.6's range:

  (top-gdb) p 0x00007ffff2f4b380 <= 0x00007ffff2f4ddb0 && 0x00007ffff2f4ddb0 < 0x00007ffff2ffc018
  $1 = true

So due to -static-lib*, we end up with _two_ copies of the
__cxa_begin_catch code:

  (top-gdb) disassemble 0x0000000000b122b0
  Dump of assembler code for function __cxa_begin_catch:
     0x0000000000b122b0 <+0>:     push   %rbx
     0x0000000000b122b1 <+1>:     mov    %rdi,%rbx
     0x0000000000b122b4 <+4>:     callq  0xb11a80 <__cxa_get_globals>
     0x0000000000b122b9 <+9>:     movabs $0xb8b1aabcbcd4d500,%rdx
  ...

  (top-gdb) disassemble 0x00007ffff2f4ddb0
  Dump of assembler code for function __cxxabiv1::__cxa_begin_catch(void*):
     0x00007ffff2f4ddb0 <+0>:     push   %rbx
     0x00007ffff2f4ddb1 <+1>:     mov    %rdi,%rbx
     0x00007ffff2f4ddb4 <+4>:     callq  0x7ffff2f4a090 <__cxa_get_globals@plt>
     0x00007ffff2f4ddb9 <+9>:     movabs $0xb8b1aabcbcd4d500,%rdx
  ...

I think we end up with libstdc++.so.6 loaded because
libsource-highlight.so depends on it.

Irrespective of whether it's a good idea to use
-static-libgcc/-static-libstdc++, GDB should not crash.  Since there
are two copies of the code, it seems right to have more than one
location.  So the fix is just to remove the assertion.

A testcase is included, which mimics the scenerio described above,
with binary linked with -static-lib{stdc++,gcc} and a shared library
that is linked normally, along with other combinations for good
measure.

gdb/ChangeLog:
2019-07-09  Pedro Alves  <palves@redhat.com>

	PR c++/15468
	* breakpoint.c (print_one_breakpoint_location): Remove
	single-location assert.

gdb/testsuite/ChangeLog:
2019-07-09  Pedro Alves  <palves@redhat.com>

	PR c++/15468
	* gdb.cp/except-multi-location-lib.cc: New.
	* gdb.cp/except-multi-location-main.cc: New.
	* gdb.cp/except-multi-location.exp: New.