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.

include/swift/Basic/Features.def

@@ -44,5 +44,6 @@ LANGUAGE_FEATURE(GlobalActors, 0, "Global actors", langOpts.EnableExperimentalCo
 LANGUAGE_FEATURE(BuiltinJob, 0, "Builtin.Job type", true)
 LANGUAGE_FEATURE(Sendable, 0, "Sendable and @Sendable", true)
 LANGUAGE_FEATURE(BuiltinContinuation, 0, "Continuation builtins", true)
+LANGUAGE_FEATURE(BuiltinTaskGroup, 0, "TaskGroup builtins", true)
 
 #undef LANGUAGE_FEATURE