[llvm] [Utils] Consolidate `LockstepReverseIterator` into own header (NFC) (PR #116657)

[Florian Hahn via llvm-commits]

fhahn wrote:

Unfortunately this breaks building on macOS with Clang and multiple build bots. I reverted the change for now in  236fa506d4f166882b63029915333d9599014644 to get things back to building.

https://github.com/llvm/llvm-project/pull/116657