Reset *THIS_CACHE in frame_unwind_try_unwinder in case of exception

Reset *THIS_CACHE in frame_unwind_try_unwinder in case of exception

It is required that unwinder->sniffer should set *this_cache to NULL if
the unwinder is not applicable or exception is thrown, so
78ac5f831692f70b841044961069e50d4ba6a76f adds clear_pointer_cleanup to set
*this_cache to NULL in case of exception in order to fix PR 14100.
https://sourceware.org/ml/gdb-patches/2012-08/msg00075.html

This patch removes that clear_pointer_cleanup, and catch all exception in
the caller of unwinder->sniffer.  In case of exception, reset *this_case.

gdb:

2017-08-11  Yao Qi  <yao.qi@linaro.org>

	* dwarf2-frame.c (clear_pointer_cleanup): Remove.
	(dwarf2_frame_cache): Remove reset_cache_cleanup.
	(dwarf2_frame_cache):
	* frame-unwind.c (frame_unwind_try_unwinder): Catch
	RETURN_MASK_ALL and set *this_case to NULL.
	* frame-unwind.h: Update comments.