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
546449938a