[Bug libstdc++/78870] Support std::filesystem on Windows

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78870

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Because nobody who uses Windows has contributed any help to make it work.
Nothing will change until somebody does that. I've written 100% of the
filesystem code so far, but I'm not going to do the Windows implementation too.