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

[Balazs Benics via llvm-commits]

steakhal wrote:

I think your reapplied attempt still has a build failure:
https://lab.llvm.org/buildbot/#/builders/38/builds/2451/steps/8/logs/stdio at line 2741
https://lab.llvm.org/buildbot/#/builders/161/builds/4812 at line 99

I would have written this comment at the PR of the reapply commit - but it didn't have a PR -.-

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