C++: Cache TNode and localFlowStep

These two elements weren't cached, which meant that local data flow was
recalculated in every query that used data flow. They are also cached in
the Java version of `DataFlowUtil.qll`.

cpp/ql/src/semmle/code/cpp/dataflow/internal/DataFlowUtil.qll

@@ -4,6 +4,7 @@
 import cpp
 private import semmle.code.cpp.dataflow.internal.FlowVar
 
+cached
 private newtype TNode =
   TExprNode(Expr e) or
   TParameterNode(Parameter p) { exists(p.getFunction().getBlock()) } or
@@ -161,6 +162,7 @@ private Variable asVariable(Node node) {
  * Holds if data flows from `nodeFrom` to `nodeTo` in exactly one local
  * (intra-procedural) step.
  */
+cached
 predicate localFlowStep(Node nodeFrom, Node nodeTo) {
   // Expr -> Expr
   exprToExprStep_nocfg(nodeFrom.asExpr(), nodeTo.asExpr())