c++: Seed imported bindings [PR 99039]

c++: Seed imported bindings [PR 99039]

As mentioned in 99040's fix, we can get inter-module using decls.  If the
using decl is the only reference to an import, we'll have failed to
seed our imports leading to an assertion failure.  The fix is
straight-forwards, check binding contents when seeding imports.

	gcc/cp/
	* module.cc (module_state::write_cluster): Check bindings for
	imported using-decls.
	gcc/testsuite/
	* g++.dg/modules/pr99039_a.C: New.
	* g++.dg/modules/pr99039_b.C: New.