Fix memclass5.C, memfriend10.C, var-templ19.C with -std=c++1z.

Fix memclass5.C, memfriend10.C, var-templ19.C with -std=c++1z.

	* constraint.cc (strictly_subsumes): New.
	* cp-tree.h: Declare it.
	* pt.c (process_partial_specialization): Use it instead of
	subsumes_constraints.
	(maybe_new_partial_specialization): Do compare null constraints.
	* search.c (lookup_member): Handle currently_open_class returning null.

From-SVN: r231350