Reverts llvm/llvm-project#145933 due to broken aarch64 buildbots.

fix #145932

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

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).

- Add a single `cmp` function to derive all comparison operators
 - Use the `friend` version of the member functions for symmetry
 - Add a `is_neg` function to factor sign extraction
 - Implement binary op through macro expansion

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079