[llvm] [LIT] replace `lit.util.mkdir` with `pathlib.Path.mkdir` (PR #163948)

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

I've verified that latest version resolves my initial problem about `*.exe`.
The title and the description are updated to reflect changes.

https://github.com/llvm/llvm-project/pull/163948