c++: requires-exprs and partial constraint subst [PR110006]

In r11-3261-gb28b621ac67bee we made tsubst_requires_expr never partially
substitute into a requires-expression so as to avoid checking its
requirements out of order during e.g. generic lambda regeneration.

These PRs however illustrate that we still sometimes do need to
partially substitute into a requires-expression, in particular when it
appears in associated constraints that we're directly substituting for
sake of declaration matching or dguide constraint rewriting.  In these
cases we're being called from tsubst_constraint during which
processing_constraint_expression_p is true, so this patch checks this
predicate to control whether we defer substitution or partially
substitute.

In turn, we now need to propagate semantic tsubst flags through
tsubst_requires_expr rather than just using tf_none, notably for sake of
dguide constraint rewriting which sets tf_dguide.

	PR c++/110006
	PR c++/112769

gcc/cp/ChangeLog:

	* constraint.cc (subst_info::quiet): Accomodate non-diagnostic
	tsubst flags.
	(tsubst_valid_expression_requirement): Likewise.
	(tsubst_simple_requirement): Return a substituted _REQ node when
	processing_template_decl.
	(tsubst_type_requirement_1): Accomodate non-diagnostic tsubst
	flags.
	(tsubst_type_requirement): Return a substituted _REQ node when
	processing_template_decl.
	(tsubst_compound_requirement): Likewise.  Accomodate non-diagnostic
	tsubst flags.
	(tsubst_nested_requirement): Likewise.
	(tsubst_requires_expr): Don't defer partial substitution when
	processing_constraint_expression_p is true, in which case return
	a substituted REQUIRES_EXPR.
	* pt.cc (tsubst_expr) <case REQUIRES_EXPR>: Accomodate
	non-diagnostic tsubst flags.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias18.C: New test.
	* g++.dg/cpp2a/concepts-friend16.C: New test.

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