C++: Fix join order in virtual dispatch with unique

The optimizer picked a terrible join order in `VirtualDispatch::DataSensitiveCall::flowsFrom()`. Telling it that `getAnOutNode()` has a unique result convinces it to join first on the `Callable`, rather than on the `ReturnKind`.

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowPrivate.qll

@@ -123,8 +123,13 @@ private class SideEffectOutNode extends OutNode {
  * `kind`.
  */
 OutNode getAnOutNode(DataFlowCall call, ReturnKind kind) {
-  result.getCall() = call and
-  result.getReturnKind() = kind
+  // There should be only one `OutNode` for a given `(call, kind)` pair. Showing the optimizer that
+  // this is true helps it make better decisions downstream, especially in virtual dispatch.
+  result =
+    unique(OutNode outNode |
+      outNode.getCall() = call and
+      outNode.getReturnKind() = kind
+    )
 }
 
 /**