PR c++/21619
	* cp-tree.h (DECL_IS_BUILTIN_CONSTANT_P): New macro.
	* parser.c (cp_parser_postfix_expression): Allow non-constant
	expressions as arguments to __builtin_constant_p.
	* tree.c (builtin_valid_in_constant_expr_p): Use
	DECL_IS_BUILTIN_CONSTANT_P.

	PR c++/21619
	* g++.dg/ext/builtin9.C: New test.

From-SVN: r100626