Make disjuncts in virtual method removal independent of class loading

The disjuncts created by virtual method removal depended
on the class loading order, causing goto-diff to think that
there was a change in the classes.

Author: peterschrammel

src/goto-programs/remove_virtual_functions.cpp

@@ -445,6 +445,8 @@ void remove_virtual_functionst::get_functions(
         has_prefix(
           id2string(b.symbol_expr.get_identifier()), "java::java.lang.Object"))
         return true;
+      else if(a.symbol_expr.get_identifier() == b.symbol_expr.get_identifier())
+        return a.class_id < b.class_id;
       else
         return a.symbol_expr.get_identifier() < b.symbol_expr.get_identifier();
     });