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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

I'm taking a look, but I'm having some doubts about the change itself, `DIBuilder::insertDbgValueIntrinsic`'s `BasicBlock *BB = InsertBefore->getParent();` (where `InsertBefore` is an iterator) doesn't seem safe, it seems like it should be valid to pass an at-the-end iterator there that cannot be dereferenced. I'll take a more in-depth look at the change and see whether I'm able to add any testing.

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