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

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

> I'm using Python 3.8.20.
Thanks for narrowing this down. 56s is still above the 47s baseline, so I don't think this is resolved yet. Can you try LIT_SUBMISSION_WINDOW values of 64, 96, 128, and 256 and share the timings? 

| LIT_SUBMISSION_WINDOW | 64 | 96 | 128 | 256 |
| - | - | - | - | - |
|  - | 56s | 58s | 61s | 72s |

so 64 is equivalent to `SUBMISSION_WINDOW_PER_WORKER=1` result earlier.

> Also, if you noticed any workers going idle with SUBMISSION_WINDOW_PER_WORKER=1 (window=64 on your machine), that would be useful to know.

I'm seeing full core utilisation with `SUBMISSION_WINDOW_PER_WORKER=1`

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