c++: concept-ids and value-dependence [PR102412]

c++: concept-ids and value-dependence [PR102412]

The problem here is that uses_template_parms returns true for all
concept-ids (even those with non-dependent arguments), so when a concept-id
is used as a default template argument then during deduction the default
argument is considered dependent even after substituting into it, which
leads to deduction failure (from type_unification_real).

This patch fixes this by implementing the resolution of CWG 2446 which
says a concept-id is dependent only if its arguments are.

	DR 2446
	PR c++/102412

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_constant_expression)
	<case TEMPLATE_ID_EXPR>: Check value_dependent_expression_p
	instead of processing_template_decl.
	* pt.c (value_dependent_expression_p) <case TEMPLATE_ID_EXPR>:
	Return true only if any_dependent_template_arguments_p.
	(instantiation_dependent_r) <case CALL_EXPR>: Remove this case.
	<case TEMPLATE_ID_EXPR>: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-nondep2.C: New test.
	* g++.dg/cpp2a/concepts-nondep3.C: New test.

(cherry picked from commit 9329344a6d81a6a5e3bd171167ebc7b158bb44f4)