C#: Fix issue in ExternalApi.

Author: michaelnebel

csharp/ql/src/Telemetry/ExternalApi.qll

@@ -8,6 +8,7 @@ private import semmle.code.csharp.dataflow.FlowSummary
 private import semmle.code.csharp.dataflow.internal.DataFlowImplCommon as DataFlowImplCommon
 private import semmle.code.csharp.dataflow.internal.DataFlowPrivate
 private import semmle.code.csharp.dataflow.internal.DataFlowDispatch as DataFlowDispatch
+private import semmle.code.csharp.dataflow.internal.FlowSummaryImpl as Impl
 private import semmle.code.csharp.dataflow.internal.TaintTrackingPrivate
 private import semmle.code.csharp.security.dataflow.flowsources.Remote
 
@@ -80,7 +81,7 @@ class ExternalApi extends DotNet::Callable {
 
   /** Holds if this API has a supported summary. */
   predicate hasSummary() {
-    this instanceof SummarizedCallable or
+    exists(Impl::Public::SummarizedCallable sc | sc.asCallable() = this) or
     defaultAdditionalTaintStep(this.getAnInput(), _)
   }