[llvm] [Support] Make shouldReverseIterate constexpr (PR #156812)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> You can make it contexpr while still keeping the macro. I'm not saying the new implementation with `if constexpr` is worse but that I don't know if it's better.

@kuhar I now retain the macro and just make the return type `constexpr`.

https://github.com/llvm/llvm-project/pull/156812