c++: requires-expressions and partial instantiation [PR96410]

c++: requires-expressions and partial instantiation [PR96410]

This patch makes tsubst_requires_expr avoid substituting into a
requires-expression when partially instantiating a generic lambda.
This is necessary in general to ensure that we always check requirements
in lexical order (as in the first testcase below).  A mechanism similar
to PACK_EXPANSION_EXTRA_ARGS is added to remember template arguments and
defer substitution of requires-expressions.

Incidentally, this change also fixes the two mentioned PRs -- the
problem there is that tsubst_requires_expr was performing semantic
checks on template trees, and some of the checks are not prepared to
handle such trees.  With this patch, tsubst_requires_expr no longer
does any semantic checking at all when processing_template_decl.

gcc/cp/ChangeLog:

	PR c++/96409
	PR c++/96410
	* constraint.cc (tsubst_requires_expr): Use REQUIRES_EXPR_PARMS
	and REQUIRES_EXPR_REQS.  Use REQUIRES_EXPR_EXTRA_ARGS,
	add_extra_args and build_extra_args to defer substitution until
	we have all the template arguments.
	(finish_requires_expr): Adjust the call to build_min so that
	REQUIRES_EXPR_EXTRA_ARGS gets set to NULL_TREE.
	* cp-tree.def (REQUIRES_EXPR): Give it a third operand.
	* cp-tree.h (REQUIRES_EXPR_PARMS, REQUIRES_EXPR_REQS,
	REQUIRES_EXPR_EXTRA_ARGS): Define.
	(add_extra_args, build_extra_args): Declare.

gcc/testsuite/ChangeLog:

	PR c++/96409
	PR c++/96410
	* g++.dg/cpp2a/concepts-lambda13.C: New test.
	* g++.dg/cpp2a/concepts-lambda14.C: New test.