c++: Register streamed-in decls when new [PR 99040]

c++: Register streamed-in decls when new [PR 99040]

With modules one can have using-decls refering to their own scope.  This
is the way to export things from the GMF or from an import.  The
problem was I was using current_ns == CP_DECL_CONTEXT (decl) to
determine whether a decl should be registered in a namespace level or
not.  But that's an inadequate check and we ended up reregistering
decls and creating a circular list.  We should be registering the decl
when first encountered -- whether we bind it is orthogonal to that.

	PR c++/99040
	gcc/cp/
	* module.cc (trees_in::decl_value): Call add_module_namespace_decl
	for new namespace-scope entities.
	(module_state::read_cluster): Don't call add_module_decl here.
	* name-lookup.h (add_module_decl): Rename to ...
	(add_module_namespace_decl): ... this.
	* name-lookup.c (newbinding_bookkeeping): Move into ...
	(do_pushdecl): ... here.  Its only remaining caller.
	(add_module_decl): Rename to ...
	(add_module_namespace_decl): ... here.  Add checking-assert for
	circularity. Don't call newbinding_bookkeeping, just extern_c
	checking and incomplete var checking.
	gcc/testsuite/
	* g++.dg/modules/pr99040_a.C: New.
	* g++.dg/modules/pr99040_b.C: New.
	* g++.dg/modules/pr99040_c.C: New.
	* g++.dg/modules/pr99040_d.C: New.