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.