c++: requires-expression folding [PR101182]

c++: requires-expression folding [PR101182]

Here we're crashing because cp_fold_function walks into the (templated)
requirements of a requires-expression outside a template, but the
folding routines aren't prepared to handle templated trees.  This patch
fixes this by making cp_fold use evaluate_requires_expr to fold a
requires-expression as a whole, which also means we no longer need to
explicitly do so during gimplification.  (Note that we delay folding
of such requires-expressions for sake of better diagnostics when one is
used as the condition of a failed static_assert.)

	PR c++/101182

gcc/cp/ChangeLog:

	* constraint.cc (evaluate_requires_expr): Adjust function comment.
	* cp-gimplify.c (cp_genericize_r) <case REQUIRES_EXPR>: Move to ...
	(cp_fold) <case REQUIRES_EXPR>: ... here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires25.C: New test.

(cherry picked from commit c06493dc30afbf65b14d783c7cd53f20877ef577)