c++: Don't strip USING_DECLs when updating local bindings [PR116748]

c++: Don't strip USING_DECLs when updating local bindings [PR116748]

Currently update_binding strips USING_DECLs too eagerly, leading to ICEs
in pop_local_decl as it can't find the decl it's popping in the binding
list.  Let's rather try to keep the original USING_DECL around.

This also means that using59.C can point to the location of the
using-decl rather than the underlying object directly; this is in the
direction required to fix PR c++/106851 (though more work is needed to
emit properly helpful diagnostics here).

	PR c++/116748

gcc/cp/ChangeLog:

	* name-lookup.cc (update_binding): Maintain USING_DECLs in the
	binding slots.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/using59.C: Update location.
	* g++.dg/lookup/using69.C: New test.

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