Remove cleanup from frame_prepare_for_sniffer

Remove cleanup from frame_prepare_for_sniffer

Currently frame_prepare_for_sniffer returns a cleanup.  This patch
changes it to return void, and exposes frame_cleanup_after_sniffer to
the caller.

Normally I would write an RAII class for this sort of thing; but
because there was just a single caller of frame_prepare_for_sniffer,
and because this caller is already using try/catch, I thought it
seemed ok to require explicit calls in this instance.

Regression tested by the buildbot.

gdb/ChangeLog
2017-10-08  Tom Tromey  <tom@tromey.com>

	* frame-unwind.c (frame_unwind_try_unwinder): Update.
	* frame.h (frame_cleanup_after_sniffer): Declare.
	(frame_prepare_for_sniffer): Return void.
	* frame.c (frame_cleanup_after_sniffer): No longer static.  Change
	type of argument.
	(frame_prepare_for_sniffer): Return void.