[clang] [llvm] [NFC][DebugInfo] Use iterators for instruction insertion in more places (PR #124291)

[Jeremy Morse via llvm-commits]

jmorse wrote:

Ta; fixed some of the clang-format errors that are on lines that I've touched, however I'm leaving the clang-format errors that are adjacent (there are some pre-existing indentation issues too it seems).

https://github.com/llvm/llvm-project/pull/124291