[Bug libstdc++/66417] std::codecvt_utf16<char32_t, 0x10FFFF, std::codecvt_mode::generate_header> generates incorrect output

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66417

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk so far.