builtins.c (fold_builtin_constant_p): Return integer_zero_node for complex expressions when cfun == 0.

	* builtins.c (fold_builtin_constant_p): Return integer_zero_node
for complex expressions when cfun == 0.
	* doc/extend.texi: Document that __builtin_constant_p can be
used in data initializers as well as functions.
	* gcc.dg/bconstp-1.c: New test.

From-SVN: r44619