[libc++] Implement LWG3940: std::expected<void, E>::value() also needs E to be...

[libc++] Implement LWG3940: std::expected<void, E>::value() also needs E to be copy constructible (#71819)

This patch includes the fix for
[LWG3940](https://cplusplus.github.io/LWG/issue3940)
(`std::expected<void, E>::value()` also needs `E` to be copy
constructible)