[concurrency] Add AsyncTask::flagAsCompleted() and behind SWIFT_TASK_PRINTF_DEBUG allow it to be used to log Task completion.

This should let people debug Task lifetime easier.

(cherry picked from commit ff6b67c)

Author: gottesmm

include/swift/ABI/Task.h

@@ -279,6 +279,10 @@ class AsyncTask : public Job {
   void flagAsSuspended();
   void flagAsSuspended_slow();
 
+  /// Flag that this task is now completed. This normally does not do anything
+  /// but can be used to locally insert logging.
+  void flagAsCompleted();
+
   /// Check whether this task has been cancelled.
   /// Checking this is, of course, inherently race-prone on its own.
   bool isCancelled() const;

stdlib/public/Concurrency/Task.cpp

@@ -219,6 +219,8 @@ static void destroyJob(SWIFT_CONTEXT HeapObject *obj) {
 }
 
 AsyncTask::~AsyncTask() {
+  flagAsCompleted();
+
   // For a future, destroy the result.
   if (isFuture()) {
     futureFragment()->destroy();

stdlib/public/Concurrency/TaskPrivate.h

@@ -357,6 +357,16 @@ inline void AsyncTask::flagAsSuspended() {
   }
 }
 
+// READ ME: This is not a dead function! Do not remove it! This is a function
+// that can be used when debugging locally to instrument when a task actually is
+// dealloced.
+inline void AsyncTask::flagAsCompleted() {
+#if SWIFT_TASK_PRINTF_DEBUG
+  fprintf(stderr, "[%lu] task completed %p\n",
+          _swift_get_thread_id(), this);
+#endif
+}
+
 inline void AsyncTask::localValuePush(const HeapObject *key,
                                       /* +1 */ OpaqueValue *value,
                                       const Metadata *valueType) {