c++: using in diagnostics [PR102987]

The expression pretty-printing code crashed on a location wrapper with no
type, and didn't know what to do with a USING_DECL.

	PR c++/102987

gcc/cp/ChangeLog:

	* error.cc (dump_expr): Handle USING_DECL.
	[VIEW_CONVERT_EXPR]: Just look through location wrapper.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/using1.C: New test.