Forget the last displayed sal when the referenced objfile is destroyed

When creating a new line-breakpoint after rerunning with a disabled or
removed breakpoint in a solib, you currently get this use-after-free
crash:

(gdb) break solib_main
Breakpoint 1 at 0x1030
(gdb) run
Starting program: /home/src/lappy/binutils-gdb.git/build/gdb/testsuite/outputs/gdb.base/solib-breakpoints-rerun/solib-breakpoints-rerun
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".

Breakpoint 1, solib_main (arg=100) at /home/src/lappy/binutils-gdb.git/build/gdb/testsuite/../../../gdb/testsuite/gdb.base/solib1.c:7
7	  int ans = arg*arg;		/* HERE */
(gdb) disable
(gdb) set confirm off
(gdb) run
Starting program: /home/src/lappy/binutils-gdb.git/build/gdb/testsuite/outputs/gdb.base/solib-breakpoints-rerun/solib-breakpoints-rerun
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".
[Inferior 1 (process 50868) exited normally]
(gdb) break 18
=================================================================
==50836==ERROR: AddressSanitizer: heap-use-after-free on address 0x7d65a543d488 at pc 0x55f022334c29 bp 0x7ffe9c7f45f0 sp 0x7ffe9c7f45e0
READ of size 8 at 0x7d65a543d488 thread T0
    #0 0x55f022334c28 in symtab::filename() const ../../gdb/symtab.h:1747
    #1 0x55f023a32b7d in create_sals_line_offset ../../gdb/linespec.c:2013
    #2 0x55f023a37937 in convert_linespec_to_sals ../../gdb/linespec.c:2294
    #3 0x55f023a3cf79 in parse_linespec ../../gdb/linespec.c:2652
    #4 0x55f023a415b2 in location_spec_to_sals ../../gdb/linespec.c:3050
    #5 0x55f023a423b0 in decode_line_full(location_spec*, int, program_space*, symtab*, int, linespec_result*, char const*, char const*) ../../gdb/linespec.c:3126
    #6 0x55f022826e1b in parse_breakpoint_sals ../../gdb/breakpoint.c:9069
    #7 0x55f0228291c2 in create_breakpoint(gdbarch*, location_spec*, char const*, int, int, char const*, bool, int, int, bptype, int, auto_boolean, breakpoint_ops const*, int, int, int, unsigned int) ../../gdb/breakpoint.c:9312
    #8 0x55f02282c298 in break_command_1 ../../gdb/breakpoint.c:9471
    #9 0x55f02282d0bb in break_command(char const*, int) ../../gdb/breakpoint.c:9541
    #10 0x55f022b05f56 in do_simple_func ../../gdb/cli/cli-decode.c:94

0x7d65a543d488 is located 904 bytes inside of 4064-byte region [0x7d65a543d100,0x7d65a543e0e0)
freed by thread T0 here:
    #0 0x7f55aa91f79d in free /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_malloc_linux.cpp:51
    #1 0x55f0221f6403 in xfree<void> ../../gdb/../gdbsupport/gdb-xfree.h:37
    #2 0x55f027b64851 in call_freefun ../../libiberty/obstack.c:103
    #3 0x55f027b66283 in _obstack_free ../../libiberty/obstack.c:280
    #4 0x55f0221f7a07 in auto_obstack::~auto_obstack() ../../gdb/../gdbsupport/gdb_obstack.h:126
    #5 0x55f023f3dd83 in objfile::~objfile() ../../gdb/objfiles.c:509
    #6 0x55f023f511d8 in std::default_delete<objfile>::operator()(objfile*) const /usr/include/c++/15.2.1/bits/unique_ptr.h:93
    #7 0x55f023f4b8cd in std::unique_ptr<objfile, std::default_delete<objfile> >::~unique_ptr() /usr/include/c++/15.2.1/bits/unique_ptr.h:399
    #8 0x55f0240b6dab in owning_intrusive_list<objfile, intrusive_base_node<objfile> >::erase(intrusive_list_iterator<objfile, intrusive_base_node<objfile> >) ../../gdb/../gdbsupport/owning_intrusive_list.h:113
    #9 0x55f0240adb68 in program_space::remove_objfile(objfile*) ../../gdb/progspace.c:202
    #10 0x55f023f3cfcf in objfile::unlink() ../../gdb/objfiles.c:409
    #11 0x55f023f40faf in objfile_purge_solibs(program_space*) ../../gdb/objfiles.c:687
    #12 0x55f02487ec19 in no_shared_libraries(program_space*) ../../gdb/solib.c:1359
    #13 0x55f024b37e1e in target_pre_inferior() ../../gdb/target.c:2474
    #14 0x55f0238ac7cd in run_command_1 ../../gdb/infcmd.c:381
    #15 0x55f0238ae438 in run_command ../../gdb/infcmd.c:510
    #16 0x55f022b05f56 in do_simple_func ../../gdb/cli/cli-decode.c:94

previously allocated by thread T0 here:
    #0 0x7f55aa920cb5 in malloc /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_malloc_linux.cpp:67
    #1 0x55f02241d780 in xmalloc ../../gdb/alloc.c:52
    #2 0x55f027b6461f in call_chunkfun ../../libiberty/obstack.c:94
    #3 0x55f027b649d1 in _obstack_begin_worker ../../libiberty/obstack.c:141
    #4 0x55f027b650c5 in _obstack_begin ../../libiberty/obstack.c:164
    #5 0x55f0221f775a in auto_obstack::auto_obstack() ../../gdb/../gdbsupport/gdb_obstack.h:123
    #6 0x55f023f39c91 in objfile::objfile(gdb::ref_ptr<bfd, gdb_bfd_ref_policy>, program_space*, char const*, enum_flags<objfile_flag>) ../../gdb/objfiles.c:257
    #7 0x55f023f3ccaa in objfile::make(gdb::ref_ptr<bfd, gdb_bfd_ref_policy>, program_space*, char const*, enum_flags<objfile_flag>, objfile*) ../../gdb/objfiles.c:392
    #8 0x55f02498b1d2 in symbol_file_add_with_addrs ../../gdb/symfile.c:1069
    #9 0x55f02498c099 in symbol_file_add_from_bfd(gdb::ref_ptr<bfd, gdb_bfd_ref_policy> const&, char const*, enum_flags<symfile_add_flag>, std::vector<other_sections, std::allocator<other_sections> >*, enum_flags<objfile_flag>, objfile*) ../../gdb/symfile.c:1156
    #10 0x55f02487255b in solib_read_symbols(solib&, enum_flags<symfile_add_flag>) ../../gdb/solib.c:660
    #11 0x55f024876c0c in solib_add(char const*, int, int) ../../gdb/solib.c:993
    #12 0x55f02487f1f8 in handle_solib_event() ../../gdb/solib.c:1399
    #13 0x55f0227df91f in bpstat_stop_status(address_space const*, unsigned long, thread_info*, target_waitstatus const&, bpstat*) ../../gdb/breakpoint.c:5962
    #14 0x55f023944fa8 in handle_signal_stop ../../gdb/infrun.c:7130
    #15 0x55f02393e2a1 in handle_inferior_event ../../gdb/infrun.c:6574
    #16 0x55f0239279f2 in fetch_inferior_event() ../../gdb/infrun.c:4713
    #17 0x55f023885fe3 in inferior_event_handler(inferior_event_type) ../../gdb/inf-loop.c:42
    #18 0x55f023af706e in handle_target_event ../../gdb/linux-nat.c:4449
    #19 0x55f027c2d2f0 in handle_file_event ../../gdbsupport/event-loop.cc:551
    #20 0x55f027c2e4ff in gdb_wait_for_event ../../gdbsupport/event-loop.cc:672

It happened because last_displayed_symtab_info of stack.c still contained
a reference to a symtab that was already freed in the 2nd run.
This fixes it by clearing last_displayed_symtab_info in the objfile
destructor, if it is pointing to that objfile.
Now setting of the 2nd breakpoint works:

(gdb) break 18
Breakpoint 2 at 0x555555555141: file /home/src/lappy/binutils-gdb.git/build/gdb/testsuite/../../../gdb/testsuite/gdb.base/so-impl-ld.c, line 18.
(gdb)

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32668
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>