C++: Update synthetic_destructor_call keysets.

Reorder the [ destructor_call, expr ] tuple.
Add a [ expr, i ] tuple.

Author: Matthew Gretton-Dann

cpp/ql/src/semmlecode.cpp.dbscheme

@@ -1061,7 +1061,8 @@ compgenerated(unique int id: @element ref);
  * destructed in reverse construction order, so for a given `element`
  * these should be called from highest to lowest `i`.
  */
-#keyset[destructor_call, element]
+#keyset[element, destructor_call]
+#keyset[element, i]
 synthetic_destructor_call(
     int element: @element ref,
     int i: int ref,