C++: Fix unintentionally changing the type of AdditionalCallTarget.viableTarget.

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

@@ -40,7 +40,7 @@ DataFlowCallable viableCallable(DataFlowCall call) {
   result = defaultViableCallable(call)
   or
   // Additional call targets
-  result =
+  result.getUnderlyingCallable() =
     any(AdditionalCallTarget additional)
         .viableTarget(call.asCallInstruction().getUnconvertedResultExpression())
 }

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

@@ -2681,5 +2681,5 @@ class AdditionalCallTarget extends Unit {
   /**
    * Gets a viable target for `call`.
    */
-  abstract DataFlowCallable viableTarget(Call call);
+  abstract Declaration viableTarget(Call call);
 }