Disable job-based dispatch integration in the back-deployed library.

Author: DougGregor

stdlib/public/Concurrency/GlobalExecutor.cpp

@@ -239,8 +239,9 @@ static void initializeDispatchEnqueueFunc(dispatch_queue_t queue, void *obj,
                                           dispatch_qos_class_t qos) {
   dispatchEnqueueFuncType func = nullptr;
 
-  // Always fall back to plain dispatch_async_f on Windows for now.
-#if !defined(_WIN32)
+  // Always fall back to plain dispatch_async_f on Windows for now, and
+  // also for back-deployed concurrency.
+#if !defined(_WIN32) && !defined(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
   if (runtime::environment::concurrencyEnableJobDispatchIntegration())
     func = reinterpret_cast<dispatchEnqueueFuncType>(
         dlsym(RTLD_NEXT, "dispatch_async_swift_job"));