C++: Cache 'toString' and 'getLocation'.

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

@@ -35,6 +35,17 @@ private module Cached {
       not Ssa::ignoreOperand(op) and exists(Ssa::getIRRepresentationOfOperand(op))
     }
 
+  cached
+  string toStringCached(Node n) {
+    result = toExprString(n)
+    or
+    not exists(toExprString(n)) and
+    result = n.toStringImpl()
+  }
+
+  cached
+  Location getLocationCached(Node n) { result = n.getLocationImpl() }
+
   cached
   newtype TContentApprox =
     TFieldApproxContent(string s) { fieldHasApproxName(_, s) } or