[Concurrency] Fix macOS build issue.

We need to use the `swift` namespace in `NonDispatchGlobalExecutor.cpp`.

rdar://135380149

Author: al45tair

stdlib/public/Concurrency/NonDispatchGlobalExecutor.cpp

@@ -34,6 +34,8 @@
 
 #include "ExecutorImpl.h"
 
+using namespace swift;
+
 SWIFT_CC(swift)
 void swift_task_enqueueGlobalImpl(SwiftJob *job) {
   assert(job && "no job provided");