[clang] [clang][nullability] allow _Nonnull etc on nullable class types (PR #82705)

[Aaron Ballman via cfe-commits]

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

LGTM though there may be a minor change to the docs that could make sense (feel free to change when landing if you agree).

https://github.com/llvm/llvm-project/pull/82705