[Bug libstdc++/90281] utf-8 encoded std::filesystem::path can not be converted to utf-16.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90281

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The problem is that I'm using codecvt_utf8<char16_t>, which converts between
UTF-8 and UCS-2 (not UTF-16). The U+1D11E is outside the basic multilingual
plane, so is not valid UCS-2.

I need to use a different codecvt facet for UTF-16.