Enable the use of ThreadPoolTaskGroup in MLIR threading helper to enable nested parallelism

The LLVM ThreadPool recently got the addition of the concept of
ThreadPoolTaskGroup: this is a way to "partition" the threadpool
into a group of tasks and enable nested parallelism through this
grouping at every level of nesting.
We make use of this feature in MLIR threading abstraction to fix a long
lasting TODO and enable nested parallelism.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D124902