[llvm] Reland "[lit] Migrate lit to ProcessPoolExecutor (#202681)" (PR #209076)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

This seems reasonable enough to me, but please let someone who ran into the issue confirm it works for them.

https://github.com/llvm/llvm-project/pull/209076