c++: dump alias-declaration scope

An alias can't be declared with a qualified-id in actual code, but in
diagnostics we want to know which scope it belongs to, and I think a
nested-name-specifier is the best way to provide that.

gcc/cp/ChangeLog:

	* error.cc (dump_decl): Check TFF_UNQUALIFIED_NAME.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-1.C: Expect qualified name.