Implement P0028R4, C++17 using attribute namespaces without repetition
	* parser.c (cp_parser_std_attribute): Add ATTR_NS argument.  Diagnose
	non-NULL ATTR_NS with scoped attribute token.  Handle non-NULL
	ATTR_NS with non-scoped attribute tokens.  Allow named ops in
	identifier after ::.
	(cp_parser_std_attribute_list): Add ATTR_NS argument, pass it down
	to cp_parser_std_attribute calls.
	(cp_parser_std_attribute_spec): Parse optional C++17
	attribute-using-prefix, adjust grammar in function comment.

	* g++.dg/cpp0x/gen-attrs-61.C: New test.
	* g++.dg/cpp1z/gen-attrs1.C: New test.

From-SVN: r240121