[llvm] [mlir] Split the llvm::ThreadPool into an abstract base class and an impleme… (PR #82094)

[David Blaikie via llvm-commits]

https://github.com/dwblaikie commented:

I'd probably use `ThreadPool` as the name for the interface, and rename the current implementation to `DefaultThreadPool` or `StdThreadPool` or `BasicThreadPool` or the like?

It might also be an opportunity to simplify the current macro and other checking for whether multithreading is enabled - we could have an UnThreadPool that's just single threaded, and use that when threading is disabled?

https://github.com/llvm/llvm-project/pull/82094