[clang] [APINotes] Upstream Sema logic to apply API Notes to decls (PR #78445)

[Saleem Abdulrasool via cfe-commits]

https://github.com/compnerd approved this pull request.

Most of the new comments are nits and small readability tweaks. I still would like to avoid the double `dyn_cast` checks, but I don't know if this is worth holding up on at that point.  Thank you for the multiple rounds on this and sorry about the delay, the length made it daunting.

https://github.com/llvm/llvm-project/pull/78445