Update PostUpdateNodes for implicit varargs slices

We don't want a post update node for the implicit varargs slice, and we
do want one for each argument which is stored in the implicit varargs
slice.

Author: owen-mc

go/ql/lib/semmle/go/dataflow/internal/DataFlowNodes.qll

@@ -732,7 +732,11 @@ module Public {
         or
         preupd = getAWrittenNode()
         or
-        preupd instanceof ArgumentNode and
+        (
+          preupd instanceof ArgumentNode and not preupd instanceof ImplicitVarargsSlice
+          or
+          preupd = any(CallNode c).getImplicitVarargsArgument(_)
+        ) and
         mutableType(preupd.getType())
       ) and
       (