c++: Fix up parsing of attributes for using-directive

c++: Fix up parsing of attributes for using-directive

As I've said earlier and added xfails in gen-attrs-76.C test,
https://eel.is/c++draft/namespace.udir#nt:using-directive
has attribute-specifier-seq[opt] at the start, not at the end before ;
as gcc is expecting.
IMHO we should continue parsing at the end the GNU attributes
because using namespace N __attribute__((strong));, while not supported
anymore, used to be supported in the past, but my code searches for
using namespace N [[gnu::strong]]; didn't reveal anything at all.

2021-08-12  Jakub Jelinek  <jakub@redhat.com>

	* parser.c (cp_parser_block_declaration): Call
	cp_parser_using_directive for C++11 attributes followed by
	using namespace tokens.
	(cp_parser_using_directive): Parse C++11 attributes at the start
	of the directive rather than at the end, only parse GNU attributes
	at the end.

	* g++.dg/lookup/strong-using.C: Add test using [[gnu::strong]]
	as well.
	* g++.dg/lookup/strong-using2.C: Likewise.
	* g++.dg/cpp0x/gen-attrs-58.C: Move alignas(int) before
	using namespace.
	* g++.dg/cpp0x/gen-attrs-59.C: Move alignas(X) before
	using namespace, add tests for alignas before semicolon.
	* g++.dg/cpp0x/gen-attrs-76.C: Remove xfails.  Add test for
	C++11 attributes on using directive before semicolon.