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

[Bill Wendling via cfe-commits]

bwendling wrote:

@nickdesaulniers I understand what you're saying, and it was addressed in the original patch. I do my best to carry the signed-ness of the variables through to the end so that the checks work out (i.e. the checks for negatives aren't removed after optimizations, which they would be if the results were unsigned).

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