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

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

> Unfortunately this breaks building on macOS with Clang and multiple build bots. I reverted the change for now in [236fa50](https://github.com/llvm/llvm-project/commit/236fa506d4f166882b63029915333d9599014644) to get things back to building.

Thank you, this (I think) broke all our builds.

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