Fix nodeGetEnclosingCallable

It wasn't updated when MkImplicitVarargsSlice was added as a branch of
TNode. This meant that it gave no result for `ImplicitVarargsSlice`s
in function calls used to initialise variables declared at file level.

Author: owen-mc

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

@@ -21,7 +21,7 @@ module Private {
   DataFlowCallable nodeGetEnclosingCallable(Node n) {
     result.asCallable() = n.getEnclosingCallable()
     or
-    (n = MkInstructionNode(_) or n = MkSsaNode(_) or n = MkGlobalFunctionNode(_)) and
+    not n instanceof FlowSummaryNode and
     not exists(n.getEnclosingCallable()) and
     result.asFileScope() = n.getFile()
     or