Fix ICE on class template argument deduction with inherited ctor.

Fix ICE on class template argument deduction with inherited ctor.

In general, when we see a dependent using-declaration we don't know whether
it names a function or not, so it doesn't get an OVERLOAD unless we see
overloads of the same name in the current class.  In the case of an
inherited constructor we could figure that out from the name, but it's
simpler to handle USING_DECL properly.

	* cp-tree.h (ovl_iterator::using_p): A USING_DECL by itself was also
	introduced by a using-declaration.

From-SVN: r273623