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

[Eli Friedman via cfe-commits]

efriedma-quic 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.

> `IncompleteArrayType` is `final`

I don't see any such marking?  I think the only reason some types are marked `final` is to ensure that TrailingObjects works correctly.

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