[llvm] [DebugInfo][RemoveDIs] Have getInsertionPtAfterDef return an iterator (PR #73149)

[Orlando Cazalet-Hyams via llvm-commits]

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

LGTM with nits and clang-formatted if it isn't already.

https://github.com/llvm/llvm-project/pull/73149