[Bug libstdc++/106589] visit rejects lambdas that do not return void

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106589

--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
variant#L1730:

  if constexpr (sizeof...(_Variants) == 0)
        return std::forward<_Visitor>(__visitor)();

In this branch, we seem to need to detect if _Result_type is void and
explicitly cast the return type to void.