[libc++][optional] LWG4497: `std::nullopt_t` should be comparable (#214975)

Implements https://wg21.link/LWG4497

Closes #189854

Also implements LWG4612: https://cplusplus.github.io/LWG/issue4612

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Co-authored-by: Mohamed Atef <mohamedatef1698@gmail.com>
Co-authored-by: Hristo Hristov <zingam@outlook.com>
GitOrigin-RevId: c31c334e11186d2c0b6a68a9f5619a4050cb87b7