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).
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