c++: Fix specialization of constrained member template.

The resolution of comment CA104 clarifies that we need to do direct
substitution of constraints in order to determine which member template
corresponds to an explicit specialization.

gcc/cp/ChangeLog
2020-05-11  Jason Merrill  <jason@redhat.com>

	Resolve C++20 NB comment CA104
	* pt.c (determine_specialization): Compare constraints for
	specialization of member template of class instantiation.