Fix viableCallable for function variables

Author: owen-mc

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

@@ -10,7 +10,7 @@ private predicate isInterfaceCallReceiver(
 ) {
   call.getReceiver() = recv and
   recv.getType().getUnderlyingType() = tp and
-  m = call.getCalleeName()
+  m = call.getACalleeIncludingExternals().asFunction().getName()
 }
 
 /** Gets a data-flow node that may flow into the receiver value of `call`, which is an interface value. */

ql/test/library-tests/semmle/go/dataflow/CallGraph/viableCallee.expected

@@ -1,6 +1,2 @@
 missingCallee
-| test.go:44:2:44:8 | call to f1 | test.go:17:1:20:1 | function declaration |
-| test.go:48:2:48:8 | call to f2 | test.go:17:1:20:1 | function declaration |
-| test.go:48:2:48:8 | call to f2 | test.go:36:1:39:1 | function declaration |
-| test.go:52:2:52:5 | call to f3 | test.go:11:1:11:28 | function declaration |
 spuriousCallee