c++: namespace reachability [PR 99344]

c++: namespace reachability [PR 99344]

This reworks namespace serializing to avoid some issues I ran into
when working on 99170.  In modules, (non-anonymous) namespaces are
strange beasts, that always have external linkage, but may have
module-specific visibility.  I still don't get the latter 100%
correct, but this is in the right direction.

	PR c++/99344
	gcc/cp/
	* module.cc (trees_out::decl_node): Small refactor.
	(depset::hash::add_binding_entity): Return true on meeting an
	import.  Set namespace's import here.
	(module_state:write_namespaces): Inform of purview too.
	(module_state:read_namespaces): Adjust.
	* name-lookup.c (implicitly_export_namespace): Delete.
	(do_pushdecl): Don't call it.
	(push_namespace): Likewise, set purview.
	(add_imported_namespace): Reorder parms.
	* name-lookup.h (add_imported_namespace): Alter param ordering.
	gcc/testsuite/
	* g++.dg/modules/namespace-2_a.C
	* g++.dg/modules/pr99344_a.C
	* g++.dg/modules/pr99344_b.C