PR python/19293 - invalidate frame cache when unwinders change
PR python/19293 notes that when a Python unwinder is disabled, the
frame cache is not invalidated. This means that disabling an unwinder
doesn't have any immediate effect -- but in my experience it's often
the case that I want to enable or disable an unwinder in order to see
what happens.
This patch adds a new gdb.invalidate_cached_frames function and
arranges for the relevant bits of library code to call it. I've only
partially documented this function, considering a warning sufficient
without going into all the reasons ordinary code should not call it.
The name of the new function was taken from a comment in frame.h next
to reinit_frame_cache.
No new test as I think the updates to the existing test are sufficient
to show that the code is working as intended.