[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 just checked and psutil is installed on my machine.

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