[Bug libstdc++/109150] std::fill should use __gnu_cxx::__is_scalar overloads for all scalars

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109150

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk for now, but I think this should be backported because the
current code on the branches is non-conforming.