[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 #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There's a silly typo that breaks the facets for big-endian targets, testing the
fix now.