c++/modules: Support unnamed namespaces in header units

c++/modules: Support unnamed namespaces in header units

A header unit may contain unnamed namespaces, and those declarations
are exported (as with any declaration in a header unit).  This patch
ensures that such declarations are correctly handled.

The change to 'make_namespace_finish' is required so that if an unnamed
namespace is first seen by an import it is correctly handled within
'add_imported_namespace'.  I don't see any particular reason why
handling of unnamed namespaces here had to be handled separately outside
that function since these are the only two callers.

gcc/cp/ChangeLog:

	* module.cc (depset::hash::add_binding_entity): Also walk
	unnamed namespaces.
	(module_state::write_namespaces): Adjust assertion.
	* name-lookup.cc (push_namespace): Move anon using-directive
	handling to...
	(make_namespace_finish): ...here.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/internal-9_a.H: New test.
	* g++.dg/modules/internal-9_b.C: New test.

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