Add flow_sensitive_info_storage and use it in gimple-fold.

Add flow_sensitive_info_storage and use it in gimple-fold.

This adds flow_sensitive_info_storage and uses it in
maybe_fold_comparisons_from_match_pd as mentioned in
https://gcc.gnu.org/pipermail/gcc-patches/2023-June/621817.html .
Since using it in maybe_fold_comparisons_from_match_pd was easy
and allowed me to test the storage earlier, I did it.

This also hides better how the flow sensitive information is
stored and only a single place needs to be updated if that
ever changes (again).

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* gimple-fold.cc (fosa_unwind): Replace `vrange_storage *`
	with flow_sensitive_info_storage.
	(follow_outer_ssa_edges): Update how to save off the flow
	sensitive info.
	(maybe_fold_comparisons_from_match_pd): Update restoring
	of flow sensitive info.
	* tree-ssanames.cc (flow_sensitive_info_storage::save): New method.
	(flow_sensitive_info_storage::restore): New method.
	(flow_sensitive_info_storage::save_and_clear): New method.
	(flow_sensitive_info_storage::clear_storage): New method.
	* tree-ssanames.h (class flow_sensitive_info_storage): New class.