c++: P0848R3 and member function templates [PR95181]

c++: P0848R3 and member function templates [PR95181]

When comparing two special member function templates to see if one hides
the other (as per P0848R3), we need to check satisfaction which we can't
do on templates.  So this patch makes add_method skip the eligibility
test on member function templates and just lets them coexist.

gcc/cp/ChangeLog:

	PR c++/95181
	* class.c (add_method): Let special member function templates
	coexist if they are not equivalently constrained, or in a class
	template.

gcc/testsuite/ChangeLog:

	PR c++/95181
	* g++.dg/concepts/pr95181.C: New test.
	* g++.dg/concepts/pr95181-2.C: New test.

Co-authored-by: Jason Merrill <jason@redhat.com>