[libcxx][optional] adds missing constexpr operations

Makes the following operations constexpr:
  * `std::swap(optional, optional)`
  * `optional(optional<U> const&)`
  * `optional(optional<U>&&)`
  * `~optional()`
  * `operator=(nullopt_t)`
  * `operator=(U&&)`
  * `operator=(optional<U> const&)`
  * `operator=(optional<U>&&)`
  * `emplace(Args&&...)`
  * `emplace(initializer_list<U>, Args&&...)`
  * `swap(optional&)`
  * `reset()`

P2231 has been accepted by plenary, with the committee recommending
implementers retroactively apply to C++20. It's necessary for us to
implement _`semiregular-box`_ and _`non-propagating-cache`_, both of
which are required for ranges (otherwise we'll need to reimplement
`std::optional` with these members `constexpr`ified).

Differential Revision: https://reviews.llvm.org/D102119