Reduce memory consumption for push/pop_access_scope.

Reduce memory consumption for push/pop_access_scope.

I was seeing memory consumption issues on the concepts-cxx2a
branch. push_scope was, surprisingly, at the top of -fmem-report, and
push_access_scope was pretty high.  Fixing them was pretty simple.

	* name-lookup.c (leave_scope): Do add class levels other than
	previous_class_level to free_binding_level.
	(invalidate_class_lookup_cache): Move from class.c, add to
	free_binding_level.
	* pt.c (saved_access_scope): Change from list to vec.

From-SVN: r273622