[Bug libstdc++/102026] The comment on the instantiation description of __variant::__gen_vtable_impl can be updated

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102026

--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
Another thing that seems noteworthy is in variant#L402:

     void _M_reset()
      {
        if (!_M_valid()) [[unlikely]]
          return;

We use C++20 attributes in the C++17 feature library, so I don’t know whether
we should use the underlined [[__unlikely__]] like we do in variant #L1310 or
it doesn’t matter.