c++: Return the underlying decl rather than the USING_DECL from update_binding [PR116913]

Users of pushdecl assume that the returned decl will be a possibly
updated decl matching the one that was passed in.  My r15-3910 change
broke this since in some cases we would now return USING_DECLs; this
patch fixes the situation.

	PR c++/116913

gcc/cp/ChangeLog:

	* name-lookup.cc (update_binding): Return the strip_using'd old
	decl rather than the binding.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/using70.C: New test.

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