[llvm] [LIT][Cygwin] Skip pre-check for existence in mkdir-p (PR #163948)

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

`mkdir_p` now simply forwards to `os.makedirs`.
Should it be inlined? It is only called from two locations in `TestRunner.py`.

`mkdir` can also be inlined as it is now only called from one location but I'm not sure if the call to `CreateDirectoryW` can be replaced with just an `os.mkdir`.

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