c++/modules: Import purview using-directives in the same module [PR122279]

c++/modules: Import purview using-directives in the same module [PR122279]

[namespace.qual] p1 says that a namespace nominated by a using-directive
is searched if the using-directive precedes that point.

[basic.lookup.general] p2 says that a declaration in a different TU
within a module purview is visible if either the declaration is
exported, or the other TU is part of the same module as the point of
lookup.  This patch implements the second half of that.

	PR c++/122279

gcc/cp/ChangeLog:

	* module.cc (depset::hash::add_namespace_entities): Seed any
	purview using-decls.
	(module_state::write_using_directives): Stream if the udir was
	exported or not.
	(module_state::read_using_directives): Add the using-directive
	if it's either exported or part of this module.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/namespace-13_b.C: Adjust expected results.
	* g++.dg/modules/namespace-13_c.C: Test non-exported
	using-directive is not used.
	* g++.dg/modules/namespace-14_a.C: New test.
	* g++.dg/modules/namespace-14_b.C: New test.
	* g++.dg/modules/namespace-14_c.C: New test.
	* g++.dg/modules/namespace-14_d.C: New test.

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