c++: Improve error location for class using-decl.

c++: Improve error location for class using-decl.

We should use the location of the using-declaration, not the location of the
class.

gcc/cp/ChangeLog:

	* class.c (handle_using_decl): Add an iloc_sentinel.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/using26.C: Adjust location.
	* g++.old-deja/g++.other/using1.C: Adjust location.