C11 DR#444 dealt with how C11 intended to allow alignment specifiers
on struct and union members, but failed to include that in the syntax.
The final resolution of that DR also allows alignment specifiers in
type names in compound literals (in order to apply an increased
alignment to the unnamed object created by the compound literal), but
not other cases of type names.

This patch implements allowing alignment specifiers in compound
literals and adds tests for the resolution of the DR (including that
they are allowed on struct and union members, which GCC already
implemented).  Because the parser has to parse the parenthesized type
name of a compound literal before it can tell that it's a compound
literal (rather than, depending on the context, a cast expression or
sizeof (type-name) or _Alignof (type-name)), this means _Alignas
specifiers are allowed syntactically in those contexts and then an
error is given once it's known to be an invalid use (whereas _Alignas
specifiers are disallowed syntactically in other contexts where type
names can occur and a compound literal is not possible).

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/c:
	* c-decl.c (build_compound_literal): Add parameter alignas_align
	and set alignment of decl if nonzero.
	* c-parser.c (c_keyword_starts_typename): Allow RID_ALIGNAS.
	(c_parser_declspecs): Allow RID_ALIGNAS to follow a type, like a
	qualifier.
	(c_parser_struct_declaration): Update syntax comment.
	(c_parser_type_name): Add alignas_ok argument and pass it to
	c_parser_declspecs.
	(c_parser_cast_expression): Pass true to c_parser_type_name and
	give error if a cast used an _Alignas specifier.
	(c_parser_sizeof_expression): Pass true to c_parser_type_name and
	give error if sizeof (type-name) used an _Alignas specifier.
	(c_parser_alignof_expression): Pass true to c_parser_type_name and
	give error if _Alignof (type-name) used an _Alignas specifier.
	(c_parser_postfix_expression_after_paren_type): Check specified
	alignment for a compound literal and pass it to
	build_compound_literal.
	* c-parser.h (c_parser_type_name): Update prototype.
	* c-tree.h (build_compound_literal): Update prototype.

gcc/testsuite:
	* gcc.dg/c11-align-7.c, gcc.dg/c11-align-8.c,
	gcc.dg/c11-align-9.c, gcc.dg/gnu11-align-1.c: New tests.
	* gcc.dg/c11-align-5.c (test): Update expected error for sizeof
	case.

From-SVN: r255482