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

[via llvm-commits]

dcandler wrote:

I couldn't find the mentioned discourse thread, but I thought I should add that this also impacted one of our downstream build machines. Ours wasn't a Mac though: this machine was running Ubuntu 22 on AArch64. However the issue is exactly the same with the tests hanging, and similarly went away after the revert, which we only just traced back to this change.

https://github.com/llvm/llvm-project/pull/202681