[llvm] Add benchmark scripts for lit performance measurement (PR #197347)

[via llvm-commits]

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prasoon054 wrote:

`os.sched_setaffinity` and `os.setpriority` are unix-only. They raise `AttributeError` here. There's no stdlib cross-platform affinity API, so Windows needs the `kernel32` calls.
`psutil` would give a cross-platform API, but it still wouldn't unify the code. Linux keeps `cset shield`. So we'd still branch per platform.

https://github.com/llvm/llvm-project/pull/197347