Don't touch frame_info objects if frame cache was reinitialized

Don't touch frame_info objects if frame cache was reinitialized

This fixes yet another bug exposed by ASAN + multi-target.exp

Running an Asan-enabled GDB against gdb.multi/multi-target.exp exposed
yet another latent GDB bug.  See here for the full log:

  https://sourceware.org/pipermail/gdb-patches/2020-July/170761.html

As Simon described, the problem is:

 - We create a new frame_info object in restore_selected_frame (by
   calling find_relative_frame)

 - The frame is allocated on the frame_cache_obstack

 - In frame_unwind_try_unwinder, we try to find an unwinder for that
   frame

 - While trying unwinders, memory read fails because the remote target
   closes, because of "monitor exit"

 - That calls reinit_frame_cache (as shown above), which resets
   frame_cache_obstack

 - When handling the exception in frame_unwind_try_unwinder, we try to
   set some things on the frame_info object (like *this_cache, which
   in fact tries to write into frame_info::prologue_cache), but the
   frame_info object is no more, it went away with the obstack.

Fix this by maintaining a frame cache generation counter.  Then in
exception handling code paths, don't touch frame objects if the
generation is not the same as it was on entry.

This commit generalizes the gdb.server/server-kill.exp testcase and
reuses it to test the scenario in question.  The new tests fail
without the GDB fix.

gdb/ChangeLog:

	* frame-unwind.c (frame_unwind_try_unwinder): On exception, don't
	touch THIS_CACHE/THIS_FRAME if the frame cache was cleared
	meanwhile.
	* frame.c (frame_cache_generation, get_frame_cache_generation):
	New.
	(reinit_frame_cache): Increment FRAME_CACHE_GENERATION.
	(get_prev_frame_if_no_cycle): On exception, don't touch
	PREV_FRAME/THIS_FRAME if the frame cache was cleared meanwhile.
	* frame.h (get_frame_cache_generation): Declare.

gdb/testsuite/ChangeLog:

	* gdb.server/server-kill.exp (prepare): New, factored out from the
	top level.
	(kill_server): New.
	(test_tstatus, test_unwind_nosyms, test_unwind_syms): New.
	(top level) : Call test_tstatus, test_unwind_nosyms, test_unwind_syms.