Do constant folding in warn_* functions.

Do constant folding in warn_* functions.

gcc/c-family/
	* c-common.c (fold_for_warn): New.
	(warn_logical_operator, warn_tautological_cmp)
	(check_function_arguments_recurse, maybe_warn_bool_compare): Use it.
gcc/cp/
	* call.c (build_new_op_1): Don't fold arguments to
	warn_logical_operator or maybe_warn_bool_compare.

From-SVN: r231198