What do you mean by “main line” – Clang itself, or everything in the LLVM/Clang repository (e.g. also Clang-Tidy)?
I’m not sure what you mean here – can you elaborate? FWIW, we’re not proposing to limit the generality or extensibility of the annotate attribute in any way. We are proposing to introduce a new general-purpose attribute annotate_type that is analogous to annotate but for use on types. (The linked RFC discusses why we’re proposing a new attribute rather than extending annotate to types.)
Currrently, we’re proposing adding the checks only to Clang-TIdy because our approach is still experimental and doing the work in Clang-Tidy has the least impact on other parts of the codebase. However, if our evaluations of the approach on large real-world codebases show that it works well, we would definitely be interested in integrating the check into Clang Static Analyzer or Clang itself.