Reuse the saved_scope structures allocated by push_to_top_level

Reuse the saved_scope structures allocated by push_to_top_level

gcc/cp/ChangeLog:

	* name-lookup.c (free_saved_scope): New free list of saved_scope
	structures.
	(push_to_top_level): Attempt to reuse a saved_scope struct
	from free_saved_scope instead of allocating a new one each time.
	(pop_from_top_level_1): Chain the now-unused saved_scope structure
	onto free_saved_scope.

From-SVN: r235351