[llvm] [lit] Migrate lit to ThreadPoolExecutor (PR #212397)

[Paul Kirth via llvm-commits]

ilovepi wrote:

@prasoon054 given the state, lets collect numbers on Linux (Windows too if you have time). Likely this was a useful experiment, but if its not buying use either a substantial performance win or code simplification, then I'm of the opinion that we probably want to just abandon this and focus on more meaningful changes, like your proposed use of asyncio. If/when you close this, if you can provide any updates or insights that would be a helpful to document why this wasn't the win we hoped for.

https://github.com/llvm/llvm-project/pull/212397