[libc++] Implement P2988R12: `std::optional<T&>` (#155202)
Resolves #148131
- Unlock `std::optional<T&>` implementation
- Allow instantiations of `optional<T(&)(...)>` and `optional<T(&)[]>`
but disables `value_or()` and `optional::iterator` + all `iterator`
related functions
- Update documentation
- Update tests
