[libc++][spaceship] Implement `operator<=>` for `optional`

[libc++][spaceship] Implement `operator<=>` for `optional`

Implements parts of **P1614R2**: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1614r2.html

- Implemented `operator<=>` for `optional`
- Updated "optional synopsis" to match the current draft https://eel.is/c++draft/optional closer
- Implemented https://cplusplus.github.io/LWG/issue3566
- Implemented https://cplusplus.github.io/LWG/issue3746

Reviewed By: #libc, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D146392