Dataflow: Add dummy DataFlowSecondLevelScope implementations.

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

java/ql/lib/semmle/code/java/dataflow/internal/DataFlowPrivate.qll

@@ -581,6 +581,8 @@ predicate knownSourceModel(Node source, string model) { sourceNode(source, _, mo
 
 predicate knownSinkModel(Node sink, string model) { sinkNode(sink, _, model) }
 
+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.