c++: Better diagnostic for decltype(auto) in C++11 [PR96103]

c++: Better diagnostic for decltype(auto) in C++11 [PR96103]

If you try to use decltype(auto) in C++11, we emit obscure

  error: expected primary-expression before 'auto'

giving the user no hint as to what's wrong.  This patch improves that
diagnostic.  Since we've been giving an error, I'm also using error().

gcc/cp/ChangeLog:

	PR c++/96103
	* parser.c (cp_parser_decltype): Print error about using decltype(auto)
	in C++11.  Check that the token following "auto" is ")".

gcc/testsuite/ChangeLog:

	PR c++/96103
	* g++.dg/cpp0x/decltype77.C: New test.