[SourceKit] Set RequestBarriersEnabled as early as possible

We were only setting `RequestBarriersEnabled` after dispatching the request to `requestQueue`. This left a brief gap where new requests could come in that wouldn’t see `RequestBarriersEnabled = true`.

Author: ahoppen

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

@@ -278,10 +278,10 @@ static void sourcekitdServer_peer_event_handler(xpc_connection_t peer,
         };
 
         if (sourcekitd::requestIsEnableBarriers(req)) {
+          RequestBarriersEnabled = true;
           dispatch_barrier_async(requestQueue, ^{
             auto Responder = std::make_shared<XPCResponder>(event, peer);
             xpc_release(event);
-            RequestBarriersEnabled = true;
             sourcekitd::sendBarriersEnabledResponse([Responder](sourcekitd_response_t response) {
               Responder->sendReply(response);
             });