[clang] Revert "Reapply "[clang][nullability] allow _Nonnull etc on nullable class types (#82705)"" (PR #87041)

[Sam McCall via cfe-commits]

sam-mccall wrote:

Sorry Douglas, and thanks for the revert.
(I missed including the new test file in the commit - I'll make sure to run through CI when relanding this!)

https://github.com/llvm/llvm-project/pull/87041