gdb: register frame_destroyed function for amd64 gdbarch

gdbarches usually register functions to check when a frame is destroyed
which is used with software watchpoints, since the expression of the
watchpoint is no longer vlaid at this point.  On amd64, this wasn't done
anymore because GCC started using CFA for variable locations instead.

However, clang doesn't use the CFA and instead relies on specifying when
an epilogue has started, meaning software watchpoints get a spurious hit
when a frame is destroyed. This patch re-adds the code to register the
function that detects when a frame is destroyed, but only uses this when
the producer is LLVM, so gcc code isn't affected. The logic that
identifies the epilogue has been factored out into the new function
amd64_stack_frame_destroyed_p_1, so the frame sniffer can call it
directly, and its behavior isn't changed.

This can also remove the XFAIL added to gdb.python/pq-watchpoint tests
that handled this exact flaw in clang.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>

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