Towards the goal of implementing __cxa_finalize (#85651) I'd like to be able to
reverse iterate over cpp::arrays such as the one used in FixedVector.

Implement the enough iterator support to be able to iterate a cpp::array in
reverse, and add tests.

Of note, reverse iterator's begin() refers to forward iterator's end() (and
vice versa). When dereferenced (operator*), the reverse iterator returns a copy
that's been pre-decremented (the underlying forward iterator is advanced).