is-optional needs to test for derived optionals

[steve-downey]

Avoids an infinite recursion in comparison operators. Fixed in standard.

namespace detail {
template <class T>
    concept is_optional = requires(const T& t) { // exposition only
    []<class U>(const optional<U>&) {}(t);
};

TODO: add test cases inheriting from optional.

Note - Bloomberg actually ran into this with moving to C++20 and spaceship synthesis of comparison operators, and one of our nullable types.