[Bug libstdc++/109941] [feat req] Add an option to mark STL types as nodiscard

[arthur.j.odwyer at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109941

--- Comment #7 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
I've split out the `std::expected` feature request specifically into bug
#119197.