NFC: Suppress an unused private member warning in ABI/Task.h.

Author: tshortli

include/swift/ABI/Task.h

@@ -170,8 +170,8 @@ static_assert(alignof(Job) == 2 * alignof(void*),
 class NullaryContinuationJob : public Job {
 
 private:
-  AsyncTask* Task;
-  AsyncTask* Continuation;
+  SWIFT_ATTRIBUTE_UNUSED AsyncTask *Task;
+  AsyncTask *Continuation;
 
 public:
   NullaryContinuationJob(AsyncTask *task, JobPriority priority, AsyncTask *continuation)

include/swift/Basic/Compiler.h

@@ -98,6 +98,12 @@
 #define SWIFT_ATTRIBUTE_NORETURN
 #endif
 
+#if __has_attribute(unused)
+#define SWIFT_ATTRIBUTE_UNUSED __attribute__((__unused__))
+#else
+#define SWIFT_ATTRIBUTE_UNUSED
+#endif
+
 #ifndef SWIFT_BUG_REPORT_URL
 #define SWIFT_BUG_REPORT_URL "https://swift.org/contributing/#reporting-bugs"
 #endif