[clang] [Clang][counted_by] Refactor __builtin_dynamic_object_size on FAMs (PR #122198)

[Nick Desaulniers via cfe-commits]

https://github.com/nickdesaulniers approved this pull request.

Thanks, I appreciate the comments about common usage patterns; they help make it easier to understand (subjectively) the 4 cases you handle in code.  I still think we may need to triple check how this extension behaves with count fields that do not get extended AND are negative at runtime (if we don't already have tests for that), but perhaps is orthogonal to this initial refactoring.  You may want to get another +1 before landing (perhaps @efriedma-quic or @rjmccall ).

https://github.com/llvm/llvm-project/pull/122198