[Bug libstdc++/83891] std::filesystem::path::is_absolute for Windows

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83891

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is still wrong, for Windows it should be:

  return has_root_name() && has_root_directory();

But I'll fix it as part of PR 78870