[lldb] [llvm] [NFC][DebugInfo] Make some block-start-position methods return iterators (PR #124287)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

Thanks, I knew it was possible (it had to be: it would be possible to create a dummy instruction, insert it at the specified point, and inspect its parent) but had not yet found the best way of doing it, I was trying to get it working with `IP.getNodePtr()->getParent()`. I'll update it to use that instead before I post anything for review.

https://github.com/llvm/llvm-project/pull/124287