Python: Remove points-to to from ControlFlowNode

Moves the existing points-to predicates to the newly added class
`ControlFlowNodeWithPointsTo` which resides in the `LegacyPointsTo`
module.

(Existing code that uses these predicates should import this module, and
references to `ControlFlowNode` should be changed to
`ControlFlowNodeWithPointsTo`.)

Also updates all existing points-to based code to do just this.

python/ql/src/Statements/StringConcatenationInLoop.ql

@@ -11,6 +11,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 predicate string_concat_in_loop(BinaryExpr b) {
   b.getOp() instanceof Add and
@@ -19,7 +20,7 @@ predicate string_concat_in_loop(BinaryExpr b) {
   |
     d.getDefinition().(DefinitionNode).getValue() = add and
     u.getAUse() = add.getAnOperand() and
-    add.getAnOperand().pointsTo().getClass() = ClassValue::str()
+    add.getAnOperand().(ControlFlowNodeWithPointsTo).pointsTo().getClass() = ClassValue::str()
   )
 }