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

[Jeremy Morse via llvm-commits]

jmorse wrote:

Prototype here https://github.com/jmorse/llvm-project/commit/8e68a722e0e2925f042cd6479f8b86578ba3042d , is that the sort of thing that would be useful? It's lacking unit test coverage right now.

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