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/Task.cpp

@@ -346,6 +346,10 @@ static AsyncTaskAndContext swift_task_create_group_future_commonImpl(
   if (flags.task_isChildTask()) {
     parent = swift_task_getCurrent();
     assert(parent != nullptr && "creating a child task with no active task");
+
+    // Inherit the priority of the parent task if unspecified.
+    if (flags.getPriority() == JobPriority::Unspecified)
+      flags.setPriority(parent->getPriority());
   }
 
   // Figure out the size of the header.