[clang] [llvm] [NFC][DebugInfo] Rewrite more call-sites to insert with iterators (PR #124288)

[Orlando Cazalet-Hyams via llvm-commits]

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

> parent-pointers can now be accessed from ilist nodes

Feels a bit of a shame we're not using this right away, but LGTM + nits

https://github.com/llvm/llvm-project/pull/124288