[clang] [CodeGen] Revamp counted_by calculations (PR #70606)

[Bill Wendling via cfe-commits]

bwendling wrote:

> I just wanted to call out that this case is interesting because the size derived from malloc and __counted_by compete each other. And __counted_by always wins. This conforms to how -fbounds-safety will see the size of objects, so sounds like we are all happy with it.

Great! :-)

As for the updating the docs and adding the intrinsic, I'd like to do that after GCC settles on their implementation. So I'd do it as a follow-up to this.

https://github.com/llvm/llvm-project/pull/70606