c++/modules: Diagnose TU-local lambdas, give mangling scope to lambdas in concepts

c++/modules: Diagnose TU-local lambdas, give mangling scope to lambdas in concepts

This fills in a hole left in r15-6378-g9016c5ac94c557 with regards to
detection of TU-local lambdas.  Now that LAMBDA_EXPR_EXTRA_SCOPE is
properly set for most lambdas we can use it to detect lambdas that are
TU-local.

CWG2988 suggests that lambdas in concept definitions should not be
considered TU-local, since they are always unevaluated and should never
be emitted. This patch gives these lambdas a mangling scope (though it
will never be actually used in name mangling).

	PR c++/116568

gcc/cp/ChangeLog:

	* cp-tree.h (finish_concept_definition): Adjust parameters.
	(start_concept_definition): Declare.
	* module.cc (depset::hash::is_tu_local_entity): Use
	LAMBDA_EXPR_EXTRA_SCOPE to detect TU-local lambdas.
	* parser.cc (cp_parser_concept_definition): Start a lambda scope
	for concept definitions.
	* pt.cc (tsubst_lambda_expr): Namespace-scope lambdas may now
	have extra scope.
	(finish_concept_definition): Split into...
	(start_concept_definition): ...this new function.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/internal-4_b.C: Remove XFAIL, add lambda alias
	testcase.
	* g++.dg/modules/lambda-9.h: New test.
	* g++.dg/modules/lambda-9_a.H: New test.
	* g++.dg/modules/lambda-9_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>