Concurrency: support newer dispatch functionality on Windows

This addresses an unintended instance where new dispatch functionality
is not used on Windows as the lookup was never performed.  This limits
the runtime to shared linking which should generally be a safe
assumption on Windows.

Author: compnerd

stdlib/public/Concurrency/DispatchGlobalExecutor.inc

@@ -84,12 +84,18 @@ 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, and
-  // also for back-deployed concurrency.
-#if !defined(_WIN32) && !defined(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
+  // Always fall back to plain dispatch_async_f for back-deployed concurrency.
+#if !defined(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
   if (runtime::environment::concurrencyEnableJobDispatchIntegration())
+#if defined(_WIN32)
+    func = reinterpret_cast<dispatchEnqueueFuncType>(
+        GetProcAddress(LoadLibraryW(L"dispatch.dll"),
+        "dispatch_async_swift_job"));
+#else
     func = reinterpret_cast<dispatchEnqueueFuncType>(
         dlsym(RTLD_NEXT, "dispatch_async_swift_job"));
+
+#endif
 #endif
 
   if (!func)