symbol: flag builtins constant_p, safe_p and warning as constexprs

index 0ceff62..cbc846b 100644
@@ -642,9 +642,10 @@ struct symbol *create_symbol(int stream, const char *name, int type, int namespa
 	return sym;
 }
 
-static int evaluate_to_integer(struct expression *expr)
+static int evaluate_to_int_const_expr(struct expression *expr)
 {
 	expr->ctype = &int_ctype;
+	expr->flags |= expr_set_flag_mask(EXPR_FLAG_INT_CONST_EXPR);
 	return 1;
 }
 

@@ -749,17 +750,17 @@ out:
 }
 
 static struct symbol_op constant_p_op = {
-	.evaluate = evaluate_to_integer,
+	.evaluate = evaluate_to_int_const_expr,
 	.expand = expand_constant_p
 };
 
 static struct symbol_op safe_p_op = {
-	.evaluate = evaluate_to_integer,
+	.evaluate = evaluate_to_int_const_expr,
 	.expand = expand_safe_p
 };
 
 static struct symbol_op warning_op = {
-	.evaluate = evaluate_to_integer,
+	.evaluate = evaluate_to_int_const_expr,
 	.expand = expand_warning
 };