[llvm] [llvm-cov][CoverageMapping] Fix LineCoverageStats incorrectly using gap region count (PR #203740)

[Maksim Levental via llvm-commits]

makslevental wrote:

FYI @chapuni I'm using claude here obviously to both understand the impl and the bug and the fix (I used claude to search for relate issues) because I'm not familiar with this part of the code base (I usually work on MLIR...). If you want me to stop using it please let me know!

https://github.com/llvm/llvm-project/pull/203740