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

[Bill Wendling via cfe-commits]

bwendling wrote:

I wrote a similar conclusion to @efriedma-quic in the email thread.

The problem with your example is that, in the absence of the `counted_by` attribute, the `__builtin_dynamic_object_size` uses the `alloc_size` attribute that's implicit on the `malloc` call.

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