c++: modules and using-directives

c++: modules and using-directives

We weren't representing 'using namespace' at all in modules, which broke
some of the <chrono> literals tests.

This only represents exported using-declarations; others should be
irrelevant to importers, as any name lookup in the imported module that
would have cared about them was done while compiling the header unit.

I experimented with various approaches to representing them; this patch
handles them in read/write_namespaces, after the namespaces themselves.  I
spent a while pondering how to deal with the depset code in order to connect
them, but then realized it would be simpler to refer to them based on their
index in the array of namespaces.

Any using-directives from an indirect import are ignored, so in an export
import, any imported using-directives are exported again.

gcc/cp/ChangeLog:

	* module.cc (module_state::write_namespaces): Write
	using-directives.
	(module_state::read_namespaces): And read them.
	* name-lookup.cc (add_using_namespace): Add overload.  Build a
	USING_DECL for modules.
	(name_lookup::search_usings, name_lookup::queue_usings)
	(using_directives_contain_std_p): Strip the USING_DECL.
	* name-lookup.h: Declare it.
	* parser.cc (cp_parser_import_declaration): Set MK_EXPORTING
	for export import.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/namespace-8_a.C: New test.
	* g++.dg/modules/namespace-8_b.C: New test.
	* g++.dg/modules/namespace-9_a.C: New test.
	* g++.dg/modules/namespace-9_b.C: New test.
	* g++.dg/modules/namespace-10_a.C: New test.
	* g++.dg/modules/namespace-10_b.C: New test.
	* g++.dg/modules/namespace-10_c.C: New test.
	* g++.dg/modules/namespace-11_a.C: New test.
	* g++.dg/modules/namespace-11_b.C: New test.
	* g++.dg/modules/namespace-11_c.C: New test.