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

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