c++/modules: Ensure deduction guides for imported types are reachable [PR120023]

c++/modules: Ensure deduction guides for imported types are reachable [PR120023]

In the linked PR, because the deduction guides depend on an imported
type, we never walk the type and so never call add_deduction_guides.
This patch ensures that we make bindings for deduction guides if we saw
any deduction guide at all.

	PR c++/120023

gcc/cp/ChangeLog:

	* module.cc (depset::hash::find_dependencies): Also call
	add_deduction_guides when walking one.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dguide-7_a.C: New test.
	* g++.dg/modules/dguide-7_b.C: New test.
	* g++.dg/modules/dguide-7_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>