Python: Add getPostUpdateNode to DataFlow::Node

as discussed in https://github.com/github/codeql/pull/5864#discussion_r634675940

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

@@ -19,7 +19,7 @@ module syntheticPreUpdateNode {
     SyntheticPreUpdateNode() { this = TSyntheticPreUpdateNode(post) }
 
     /** Gets the node for which this is a synthetic pre-update node. */
-    Node getPostUpdateNode() { result = post }
+    override NeedsSyntheticPreUpdateNode getPostUpdateNode() { result = post }
 
     override string toString() { result = "[pre " + post.label() + "] " + post.toString() }
 

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -123,6 +123,9 @@ class Node extends TNode {
    * Gets a local source node from which data may flow to this node in zero or more local data-flow steps.
    */
   LocalSourceNode getALocalSource() { result.flowsTo(this) }
+
+  /** Gets the `PostUpdateNode` for this node, if any. */
+  PostUpdateNode getPostUpdateNode() { result.getPreUpdateNode() = this }
 }
 
 /** A data-flow node corresponding to an SSA variable. */