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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I think the changes look reasonable but I'd appreciate final sign-off from @efriedma-quic or @rjmccall given this is all codegen work.

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