Resolves#174350
- Several issues were found in the current implementation of
`optional<T&>`
- `value()`, `operator*()`, `and_then()`, `transform()`, `operator->()`
still provided their ref-qualified versions for rvalues and `const&`.
- Using the listed methods on an rvalue `optional<T&>` would cause a
compile failure due to a mismatch in return types.
- On the latter, `operator*`, `operator->` would return `const` for a
`optional<T&>`, which is an incorrect deep const.
- A few constructors were missing (`optional<U>&`), and most
constructors relevant to `optional<T&>` were missing `noexcept`
- Constructors and `emplace` were not correctly constructing a `T&` as
specified in _`convert-ref-init-val`_
- Also corrects the behavior of `value_or` which should return
`remove_cv_t<T>` (in our case `decay_t<_Tp>`)
- Add several test cases to verify behavior, update `value_or` tests
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
