Dataflow: Add support for flow state.

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

@@ -3,6 +3,25 @@ private import DataFlowImplSpecific::Public
 import Cached
 
 module DataFlowImplCommonPublic {
+  /** A state value to track during data flow. */
+  abstract class FlowState extends string {
+    bindingset[this]
+    FlowState() { any() }
+  }
+
+  /**
+   * The default state, which is used when the state is unspecified for a source
+   * or a sink.
+   */
+  class FlowStateEmpty extends FlowState {
+    FlowStateEmpty() { this = "" }
+  }
+
+  // Dummy value to avoid misoptimizations when FlowState has size 1.
+  private class FlowStateDummy extends FlowState {
+    FlowStateDummy() { this = "dummy" }
+  }
+
   private newtype TFlowFeature =
     TFeatureHasSourceCallContext() or
     TFeatureHasSinkCallContext() or