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

[Aaron Ballman via cfe-commits]

AaronBallman 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. WDYT?

To clarify the problem, qualifiers are stripped from the return type in C when forming the function type itself (see C23 6.7.6.3p4).

> We haven't experimented these alternative designs so I'm curious about @AaronBallman's thoughts.

I think that encoding this into `Qualifiers` would be difficult because that's a 32-bit opaque value and I think counted_by needs to store more information than we have room for: https://github.com/llvm/llvm-project/blob/b0c6fc81fe132f20eed38bd836620dfcb5ac17e9/clang/include/clang/AST/Type.h#L147

I think `AttributedType` might be a reasonable way forward. That's how we model nullability qualifiers, for example: https://github.com/llvm/llvm-project/blob/0aacd44a4698da012ee0704815123b28f2a06d0f/clang/lib/AST/Type.cpp#L4514 and https://github.com/llvm/llvm-project/blob/b0c6fc81fe132f20eed38bd836620dfcb5ac17e9/clang/include/clang/AST/Type.h#L5189 and counted_by feels roughly similar in terms of behavior (not able to overload based on presence/absence of the attribute, etc).

CC @erichkeane as attributes code owner.

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