PR c++/54955 - Fail to parse alignas expr at the beginning of a declaration

PR c++/54955 - Fail to parse alignas expr at the beginning of a declaration

In this PR, g++ embarrassingly fails to parse the simple alignas
expression below:

    alignas(double) int f;

even though the simple-declaration production in Clause 7 suggests
otherwise.

Fixed thus and tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp

	PR c++/54955
	* parser.c (cp_nth_tokens_can_be_std_attribute_p): Recognize the
	'Alignas' keyword as the beginning of a c++11 attribute specifier.
	Update the comment of the function.
	(cp_next_tokens_can_be_gnu_attribute_p): Update the comment of the
	function.

gcc/testsuite/

	PR c++/54955
	* g++.dg/cpp0x/gen-attrs-48-2.C: New test.

From-SVN: r193029