[llvm] [clang] [Clang] Correct __builtin_dynamic_object_size for subobject types (PR #78526)

[Richard Smith via cfe-commits]

zygoloid wrote:

It might be academic at this point, but for what it's worth, `__builtin_dynamic_object_size` is not a GCC builtin that clang copied, it's [a clang builtin](https://reviews.llvm.org/D56760) that GCC copied.

https://github.com/llvm/llvm-project/pull/78526