C++: Update ppReprType for C++ IR dataflow

I forgot to do this in b1be123e31. Without this change, we suffix
` : void` on very step of an IR path explanation.

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

@@ -154,7 +154,7 @@ Type getErasedRepr(Type t) {
 }
 
 /** Gets a string representation of a type returned by `getErasedRepr`. */
-string ppReprType(Type t) { result = t.toString() }
+string ppReprType(Type t) { none() } // stub implementation
 
 /**
  * Holds if `t1` and `t2` are compatible, that is, whether data can flow from