c++: constraints and explicit instantiation [PR96164]

When considering to instantiate a member of a class template as part of
an explicit instantiation of the class template, we need to first check
the member's constraints before proceeding with the instantiation of the
member.

gcc/cp/ChangeLog:

	PR c++/96164
	* constraint.cc (constraints_satisfied_p): Return true if
	!flags_concepts.
	* pt.c (do_type_instantiation): Update a paragraph taken from
	[temp.explicit] to reflect the latest specification.  Don't
	instantiate a member with unsatisfied constraints.

gcc/testsuite/ChangeLog:

	PR c++/96164
	* g++.dg/cpp2a/concepts-explicit-inst5.C: New test.