Fix core-file -> detach -> crash (corefiles/29275)

Fix core-file -> detach -> crash (corefiles/29275)

After loading a core file, you're supposed to be able to use "detach"
to unload the core file.  That unfortunately regressed starting with
GDB 11, with these commits:

 1192f124a308 - gdb: generalize commit_resume, avoid commit-resuming when threads have pending statuses
 408f66864a1a - detach in all-stop with threads running

resulting in a GDB crash:

 ...
 Thread 1 "gdb" received signal SIGSEGV, Segmentation fault.
 0x0000555555e842bf in maybe_set_commit_resumed_all_targets () at ../../src/gdb/infrun.c:2899
 2899          if (proc_target->commit_resumed_state)
 (top-gdb) bt
 #0  0x0000555555e842bf in maybe_set_commit_resumed_all_targets () at ../../src/gdb/infrun.c:2899
 #1  0x0000555555e848bf in scoped_disable_commit_resumed::reset (this=0x7fffffffd440) at ../../src/gdb/infrun.c:3023
 #2  0x0000555555e84a0c in scoped_disable_commit_resumed::reset_and_commit (this=0x7fffffffd440) at ../../src/gdb/infrun.c:3049
 #3  0x0000555555e739cd in detach_command (args=0x0, from_tty=1) at ../../src/gdb/infcmd.c:2791
 #4  0x0000555555c0ba46 in do_simple_func (args=0x0, from_tty=1, c=0x55555662a600) at ../../src/gdb/cli/cli-decode.c:95
 #5  0x0000555555c112b0 in cmd_func (cmd=0x55555662a600, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:2514
 #6  0x0000555556173b1f in execute_command (p=0x5555565c5916 "", from_tty=1) at ../../src/gdb/top.c:699

The code that crashes looks like:

 static void
 maybe_set_commit_resumed_all_targets ()
 {
   scoped_restore_current_thread restore_thread;

   for (inferior *inf : all_non_exited_inferiors ())
     {
       process_stratum_target *proc_target = inf->process_target ();

       if (proc_target->commit_resumed_state)
           ^^^^^^^^^^^

With 'proc_target' above being null.  all_non_exited_inferiors filters
out inferiors that have pid==0.  We get here at the end of
detach_command, after core_target::detach has already run, at which
point the inferior _should_ have pid==0 and no process target.  It is
clear it no longer has a process target, but, it still has a pid!=0
somehow.

The reason the inferior still has pid!=0, is that core_target::detach
just unpushes, and relies on core_target::close to actually do the
getting rid of the core and exiting the inferior.  The problem with
that is that detach_command grabs an extra strong reference to the
process stratum target, so the unpush_target inside
core_target::detach doesn't actually result in a call to
core_target::close.

Fix this my moving the cleaning up the core inferior to a shared
routine called by both core_target::close and core_target::detach.  We
still need to cleanup the inferior from within core_file::close
because there are paths to it that want to get rid of the core without
going through detach.  E.g., "core-file" -> "run".

This commit includes a new test added to gdb.base/corefile.exp to
cover the "core-file core" -> "detach" scenario.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29275

Change-Id: Ic42bdd03182166b19f598428b0dbc2ce6f67c893