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

[Douglas Yung via llvm-commits]

dyung wrote:

> Could you please check whether `psutil` is installed? The current implementation for cleaning up processes after "Ctrl+C" uses `psutil` to terminate the process tree on `SIGINT`, or, for that matter any python exception. See here:
> 
> https://github.com/llvm/llvm-project/blob/2d12290550aca7e78a4575b4b09f75a3bc29576b/llvm/utils/lit/lit/util.py#L407-L440

I do believe it is installed as I do not see the 

> I have reverted this change for now, based on my discussion with @ilovepi @boomanaiden154 and @petrhosek Check here: #206138 We haven't been able to reproduce this on our Mac machines, both of which run on macOS 26.5.1. I will open a discourse thread where we can discuss how to get a fix for this.

Thanks! Tag me on the thread and I'll see what I can do to help get this fixed.

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