c++: Propagate using decls from partitions [PR114868]

c++: Propagate using decls from partitions [PR114868]

The modules code currently neglects to set OVL_USING_P on the dependency
created for a using-decl, which causes it not to remember that the
OVL_EXPORT_P flag had been set on it when emitted from the primary
interface unit. This patch ensures that it occurs.

	PR c++/114868

gcc/cp/ChangeLog:

	* module.cc (depset::hash::add_binding_entity): Propagate
	OVL_USING_P for using-declarations.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-15_a.C: New test.
	* g++.dg/modules/using-15_b.C: New test.
	* g++.dg/modules/using-15_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>