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

[via cfe-commits]

cor3ntin wrote:

Would it not be be better to transform the result to a windows path, such as with `pathlib.WindowsPath(toplevel).str()` ?

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