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

[Jeremy Morse via llvm-commits]

================
@@ -448,6 +455,9 @@ BasicBlock::const_iterator BasicBlock::getFirstNonPHIOrDbgOrAlloca() const {
       ++InsertPt;
     }
   }
+
+  // Signal that this comes after any debug records.
+  InsertPt.setHeadBit(false);
----------------
jmorse wrote:

The other two call-sites always generate iterators with a false head bit anyway, but there's a path from `getFirstNonPHIIt` to this line without InsertPt being assigned a different iterator. `getFirstNonPHIIt` will set the head bit, therefore we have to explicitly clear it here to ensure the position is "after" any debug records.

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