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

[Yeoul Na via cfe-commits]

https://github.com/rapidsna commented:

Thanks for addressing comments! LGTM. You may still need to wait on Clang code owners review though.

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