allow comparing Optional<T> with none

allow comparing Optional<T> with none

Summary:
`std::optional` supports comparing with `std::nullopt`, so
`folly::Optional` should do the same with `folly::none`.
This also involves marking hasValue() noexcept to be the same as `std::optional`.

Reviewed By: yfeldblum, WillerZ

Differential Revision: D5617825

fbshipit-source-id: a4b863dd61c3a86223fb21a5b7759e7c295fd272