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

[Bill Wendling via cfe-commits]

bwendling wrote:

> 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.

I'll look into test cases with negative values. Thanks!

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