Clean up the TaskGroup ABI:

- stop storing the parent task in the TaskGroup at the .swift level
- make sure that swift_taskGroup_isCancelled is implied by the parent
  task being cancelled
- make the TaskGroup structs frozen
- make the withTaskGroup functions inlinable
- remove swift_taskGroup_create
- teach IRGen to allocate memory for the task group
- don't deallocate the task group in swift_taskGroup_destroy

To achieve the allocation change, introduce paired create/destroy builtins.

Furthermore, remove the _swiftRetain and _swiftRelease functions and
several calls to them.  Replace them with uses of the appropriate builtins.
I should probably change the builtins to return retained, since they're
working with a managed type, but I'll do that in a separate commit.

stdlib/public/Concurrency/TaskGroup.cpp

@@ -470,18 +470,11 @@ static void swift_taskGroup_initializeImpl(TaskGroup *group) {
   assert(impl == record && "the group IS the task record");
 
   // ok, now that the group actually is initialized: attach it to the task
-  swift_task_addStatusRecord(record);
-}
+  bool notCancelled = swift_task_addStatusRecord(record);
 
-// =============================================================================
-// ==== create -----------------------------------------------------------------
-SWIFT_CC(swift)
-static TaskGroup *swift_taskGroup_createImpl() {
-  // TODO: John suggested we should rather create from a builtin, which would allow us to optimize allocations even more?
-  void *allocation = swift_task_alloc(sizeof(TaskGroup));
-  auto group = reinterpret_cast<TaskGroup *>(allocation);
-  swift_taskGroup_initialize(group);
-  return group;
+  // If the task has already been cancelled, reflect that immediately in
+  // the group status.
+  if (!notCancelled) impl->statusCancel();
 }
 
 // =============================================================================
@@ -514,9 +507,6 @@ void TaskGroupImpl::destroy() {
     taskDequeued = readyQueue.dequeue(item);
   }
   mutex.unlock(); // TODO: remove fragment lock, and use status for synchronization
-
-  // TODO: get the parent task, do we need to store it?
-  swift_task_dealloc(this);
 }
 
 // =============================================================================