[SourceKit] Fix an xpc_object leak

Make sure we release the `event` for cancellation
requests. This also ensures the `xpc_transaction`
for the request is ended.

Author: hamishknight

tools/SourceKit/tools/sourcekitd/bin/XPC/Service/XPCService.cpp

@@ -308,13 +308,16 @@ static void sourcekitdServer_peer_event_handler(xpc_connection_t peer,
         dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), ^{
           sourcekitd::cancelRequest(/*CancellationToken=*/cancelToken);
         });
+        xpc_release(event);
       } else if (SourceKitCancellationToken cancelToken =
                      reinterpret_cast<SourceKitCancellationToken>(
                          xpc_dictionary_get_uint64(
                              event, xpc::KeyDisposeRequestHandle))) {
         sourcekitd::disposeCancellationToken(/*CancellationToken=*/cancelToken);
+        xpc_release(event);
       } else {
         assert(false && "unexpected message");
+        xpc_release(event);
       }
     });
   }