[clang] [Clang] Fix __builtin_dynamic_object_size off by 4 (PR #111015)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I'm not sure what you think you're proving with that example; without the counted_by, we can't compute the size of the struct, so we just fall back to returning the size of the allocation.

https://github.com/llvm/llvm-project/pull/111015