[libc++] Implement LWG3843 (std::expected<T,E>::value() & assumes E is copy constructible)

Implement LWG3843 (std::expected<T,E>::value() & assumes E is copy constructible)
https://wg21.link/LWG3843

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D154110