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.

lib/SILOptimizer/Transforms/AccessEnforcementReleaseSinking.cpp

@@ -144,6 +144,8 @@ static bool isBarrier(SILInstruction *inst) {
     case BuiltinValueKind::GetCurrentAsyncTask:
     case BuiltinValueKind::GetCurrentExecutor:
     case BuiltinValueKind::AutoDiffCreateLinearMapContext:
+    case BuiltinValueKind::CreateTaskGroup:
+    case BuiltinValueKind::DestroyTaskGroup:
       return false;
 
     // Handle some rare builtins that may be sensitive to object lifetime