expr2c/{r,w,rw}_ok: add a dedicated method

This will ensure stable output even when the internals of the expression
change (as is done in follow-up commits).

Author: tautschnig

src/ansi-c/expr2c_class.h

@@ -28,6 +28,7 @@ class qualifierst;
 class namespacet;
 class r_or_w_ok_exprt;
 class pointer_in_range_exprt;
+class r_or_w_ok_exprt;
 
 class expr2ct
 {