[llvm] [Support] Use "if constexpr" in shouldReverseIterate (PR #156812)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

Is this really a simplification? This is a macro, so I'm not sure what the benefit is. Is it for better IDE/lsp support?

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