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/AST/Builtins.cpp

@@ -1481,6 +1481,18 @@ static ValueDecl *getResumeContinuationThrowing(ASTContext &ctx,
                             _void);
 }
 
+static ValueDecl *getCreateTaskGroup(ASTContext &ctx, Identifier id) {
+  return getBuiltinFunction(ctx, id, _thin,
+                            _parameters(),
+                            _rawPointer);
+}
+
+static ValueDecl *getDestroyTaskGroup(ASTContext &ctx, Identifier id) {
+  return getBuiltinFunction(ctx, id, _thin,
+                            _parameters(_rawPointer),
+                            _void);
+}
+
 static ValueDecl *getBuildSerialExecutorRef(ASTContext &ctx, Identifier id) {
   // TODO: restrict the generic parameter to the SerialExecutor protocol
   return getBuiltinFunction(ctx, id, _thin,
@@ -2728,6 +2740,12 @@ ValueDecl *swift::getBuiltinValueDecl(ASTContext &Context, Identifier Id) {
   case BuiltinValueKind::DestroyDefaultActor:
     return getDefaultActorInitDestroy(Context, Id);
 
+  case BuiltinValueKind::CreateTaskGroup:
+    return getCreateTaskGroup(Context, Id);
+
+  case BuiltinValueKind::DestroyTaskGroup:
+    return getDestroyTaskGroup(Context, Id);
+
   case BuiltinValueKind::ResumeNonThrowingContinuationReturning:
   case BuiltinValueKind::ResumeThrowingContinuationReturning:
     return getResumeContinuationReturning(Context, Id);