[llvm] [Support][Windows] disk_space handle unicode paths (PR #170716)

[Saleem Abdulrasool via llvm-commits]

https://github.com/compnerd approved this pull request.

Thanks for this cleanup! All FS operations really should use the wide variant, and the cleanup with the `QuadPart` is nice.

https://github.com/llvm/llvm-project/pull/170716