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

[jhasse at bixense dot com]

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

--- Comment #8 from Jan Niklas Hasse <jhasse at bixense dot com> ---
I'm not interested in doing all this work again from scratch, especially since
working with the Windows API is a pain.