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

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

https://lab.llvm.org/buildbot/#/builders/123/builds/30061
https://lab.llvm.org/buildbot/#/builders/210/builds/4853/steps/12/logs/stdio

Some bots fail in `litsupport/test.py` due to `lit.util.mkdir_p` missing, but it seems an external code.
What should I do for this? Restoring back `lit.util.mkdir_p` and `lit.util.mkdir` with delegating to pathlib is enough?

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