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

[Prasoon Kumar via llvm-commits]

prasoon054 wrote:

Yes. I have the numbers for `check-llvm` on macOS and Linux:

macOS 26.5.2 (M5):
PPE: 206.97 sec
`multiprocessing.pool`: 205.85 sec

Linux 6.8.0 (Ubuntu 24.04, Intel 13450HX):
PPE: 104.383 sec
`multiprocessing.pool`: 104.427 sec

These numbers are the average across 5 runs after warming it up twice.

I'll be able to add numbers for Windows later today, since I hadn't checked it there previously. I'll be running Windows on the same hardware as Linux. I'll add those to the PR as well.

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