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

[Jeremy Morse via llvm-commits]

https://github.com/jmorse commented:

Adjusted two call sites, see comments on the others. I've also added an extra unwrap to CoroSplit.cpp due to it not compiling -- this is going to be reworked in #124288 anyway.

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