[clang] Turn 'counted_by' into a type attribute and parse it into 'CountAttributedType' (PR #78000)

[Yeoul Na via cfe-commits]

rapidsna wrote:

> One possibility would be to use a type qualifier? It has basically the kind of properties you want... the underlying type is > still there, and places that strip qualifiers will automatically do the right thing in a lot of cases. It might require a bit more > work to handle it in various places in semantic analysis, though.

Thanks @efriedma-quic! We also thought about using qualifiers before but it's tricky because we would like the bounds annotations to be able to apply to function return types and parameters. WDYT?

https://github.com/llvm/llvm-project/pull/78000