C++: Silence two more QL compiler warnings

One was for an unused parameter (a deliberate CP of `Type` x
`VoidType`), and one was for use of a deprecated predicate.

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

@@ -128,6 +128,7 @@ predicate readStep(Node node1, Content f, Node node2) {
  * numeric conversions, and otherwise the erasure is used.
  */
 RefType getErasedRepr(Type t) {
+  t = t and // silence compiler warning
   result instanceof VoidType // stub implementation
 }
 

cpp/ql/src/semmle/code/cpp/ir/internal/PrintIRImpl.qll

@@ -83,7 +83,8 @@ class PrintableFunctionIR extends PrintableIRNode, TPrintableFunctionIR {
   override int getOrder() {
     this = rank[result + 1](PrintableFunctionIR orderedFunc, Location location |
       location = orderedFunc.getFunctionIR().getLocation() |
-      orderedFunc order by location.getFile().getURL(), location.getStartLine(),
+      orderedFunc order by location.getFile().getAbsolutePath(),
+        location.getStartLine(),
         location.getStartColumn(), orderedFunc.getLabel()
     )
   }