Work around lack of qos_class_self in non-Darwin libdispatch

(cherry picked from commit fbc6237)

Author: DougGregor

stdlib/public/Concurrency/Actor.cpp

@@ -264,7 +264,11 @@ JobPriority swift::swift_task_getCurrentThreadPriority() {
   if (isExecutingOnMainThread())
     return JobPriority::UserInitiated;
 
+#if defined(__APPLE__)
   return static_cast<JobPriority>(qos_class_self());
+#else
+  return JobPriority::Unspecified;
+#endif
 }
 
 SWIFT_CC(swift)

test/Concurrency/Runtime/async.swift

@@ -56,7 +56,10 @@ if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
   asyncTests.test("GlobalDispatchQueue") {
     DispatchQueue.global(qos: .utility).async {
       async {
+#if (os(macOS) || os(iOS) || os(tvOS) || os(watchOS))
+        // Non-Darwin platforms currently lack qos_class_self().
         assert(Task.currentPriority == .utility)
+#endif
       }
     }
     sleep(1)