[clang] [llvm] [polly] [NFC][DebugInfo] Use iterator moveBefore at many call-sites (PR #123583)

[Jeremy Morse via llvm-commits]

jmorse wrote:

If it's alright, I'll deal with the comment and different-arg-name matters in a follow up commit -- I've got rewrite those areas for deprecation anyway. Also force pushing a rebased commit because I don't trust what github will do with merging main into this.

https://github.com/llvm/llvm-project/pull/123583