Dataflow: Add dummy DataFlowSecondLevelScope implementations.

These could be an empty type, but Unit was available and it probably
doesn't matter.

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -1087,6 +1087,8 @@ predicate knownSinkModel(Node sink, string model) {
   sink = ModelOutput::getASinkNode(_, model).asSink()
 }
 
+class DataFlowSecondLevelScope = Unit;
+
 /**
  * Holds if flow is allowed to pass from parameter `p` and back to itself as a
  * side-effect, resulting in a summary from `p` to itself.