c++: Refinements to "more constrained".

c++: Refinements to "more constrained".

P2113 from the last C++ meeting clarified that we only compare constraints
on functions or function templates that have equivalent template parameters
and function parameters.

I'm not currently implementing the complicated handling of reversed
comparison operators here; thinking about it now, it seems like a lot of
complexity to support a very weird usage.  If I write two similar comparison
operators to be distinguished by their constraints, why would I write one
reversed?  If they're two unrelated operators, they're very unlikely to be
similar enough for the complexity to help.  I've started a discussion on the
committee reflector about changing these rules.

This change breaks some greedy_ops tests in libstdc++ that were relying on
comparing constraints on unrelated templates, which seems pretty clearly
wrong, so I'm removing those tests for now.

gcc/cp/ChangeLog:

	* call.c (joust): Only compare constraints for non-template
	candidates with matching parameters.
	* pt.c (tsubst_pack_expansion): Fix getting a type parameter
	pack.
	(more_specialized_fn): Only compare constraints for candidates with
	matching parameters.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-return-req1.C: Expect error.
	* g++.dg/cpp2a/concepts-p2113a.C: New test.
	* g++.dg/cpp2a/concepts-p2113b.C: New test.

libstdc++-v3/ChangeLog:

	* testsuite/24_iterators/move_iterator/rel_ops_c++20.cc:
	Remove greedy_ops tests.
	* testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc:
	Remove greedy_ops tests.