[clang] [Clang][counted_by] Add support for 'counted_by' on struct pointers (PR #137250)

[Kees Cook via cfe-commits]

kees wrote:

This is looking good, but needs to support `void *` struct members (they are treated as byte-sized for pointer arithmetic and `memcpy`). GCC has added support for this. Otherwise, yes, this is building for Linux with some (non-`void *`) annotations added and is operating correctly.

https://github.com/llvm/llvm-project/pull/137250