[Bug libstdc++/85951] make_signed and make_unsigned are incorrect for wchar_t, char16_t, and char32_t

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85951

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for trunk and 8.2 so far.