c++: Export usings referring to global module fragment [PR109679]

This patch stops 'add_binding_entity' from ignoring all names in the
global module fragment, since they should still be exported if named
in an exported using-declaration.

	PR c++/109679

gcc/cp/ChangeLog:

	* module.cc (depset::hash::add_binding_entity): Don't skip names
	in the GMF if they've been exported with a using declaration.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-11.h: New test.
	* g++.dg/modules/using-11_a.C: New test.
	* g++.dg/modules/using-11_b.C: New test.

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