Allow comments within _Pragma(). Fixes #2231.

test/val/bug2231.c

@@ -0,0 +1,10 @@
+_Pragma("message/*Comment1*/ ( /*Comment2*/\"test message\" /*Comment3*/)")
+/* We have no pragma without parenthesis but if there would be one, the
+** following should also work:
+*/
+/* _Pragma("once// Comment") */
+
+int main(void)
+{
+    return 0;
+}