[llvm] [ADT] Add `llvm::reverse_conditionally()` iterator (PR #171040)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar approved this pull request.

LGTM but you may want to leave it open for a day or two to see if someone else has comments

https://github.com/llvm/llvm-project/pull/171040