Even though these two using-declarations have the same effect, they are not
the same declaration, and we don't need to work to treat them as the same
like we do for typedefs. If we did need to, we would need to handle them
specially in iterative_hash_template_arg as well as here.
* tree.c (cp_tree_equal): Always return false for USING_DECL.
From-SVN: r269777
