[Bug libstdc++/69703] char16_t conversions broken in filesystem::path

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69703

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh, this is PR 66855 again, but for codecvt_utf8 this time.