[clang] [Clang] Implement the 'counted_by' attribute (PR #76348)

[Yeoul Na via cfe-commits]

rapidsna wrote:

> The return value of -1 means "I don't know how large this is". 

@kees Oh, I see. I did not know such the convention. Is it documented somewhere?

https://github.com/llvm/llvm-project/pull/76348