[Bug libstdc++/98978] Consider packing _M_Engaged in the tail padding of T in optional<>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98978

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
If we were going to do this, we could also make std::optional<bool> occupy a
single byte, using one bit for the value and one for the engaged flag.