[clang] [clang-format] convert path to Windows path if user is using a MSYS2 shell (PR #111526)

[Owen Pan via cfe-commits]

owenca wrote:

> > I personally use Cygwin and it's git understands unix paths is that not the case for msys2
> 
> Git can understand (and will return a) unix path, but it seems MSYS2's python won't accept unix path. It fails at `os.chdir(toplevel)`, and `toplevel` is a unix path.

Isn't this a Python/MSYS2 bug? Is it related to #114078?

https://github.com/llvm/llvm-project/pull/111526