Revert "Reapply "[clang][nullability] allow _Nonnull etc on nullable class...

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

This reverts commit bbbcc1d9.

This change is causing the following build bots to fail due to a missing
header file:
- https://lab.llvm.org/buildbot/#/builders/188/builds/43765
- https://lab.llvm.org/buildbot/#/builders/176/builds/9428
- https://lab.llvm.org/buildbot/#/builders/187/builds/14696
- https://lab.llvm.org/buildbot/#/builders/186/builds/15551
- https://lab.llvm.org/buildbot/#/builders/182/builds/9413
- https://lab.llvm.org/buildbot/#/builders/245/builds/22507
- https://lab.llvm.org/buildbot/#/builders/258/builds/16026
- https://lab.llvm.org/buildbot/#/builders/249/builds/17221
- https://lab.llvm.org/buildbot/#/builders/38/builds/18566
- https://lab.llvm.org/buildbot/#/builders/214/builds/11735
- https://lab.llvm.org/buildbot/#/builders/231/builds/21947
- https://lab.llvm.org/buildbot/#/builders/230/builds/26675
- https://lab.llvm.org/buildbot/#/builders/57/builds/33922
- https://lab.llvm.org/buildbot/#/builders/124/builds/10311
- https://lab.llvm.org/buildbot/#/builders/109/builds/86173
- https://lab.llvm.org/buildbot/#/builders/280/builds/1043
- https://lab.llvm.org/buildbot/#/builders/283/builds/440
- https://lab.llvm.org/buildbot/#/builders/247/builds/16034
- https://lab.llvm.org/buildbot/#/builders/139/builds/62423
- https://lab.llvm.org/buildbot/#/builders/216/builds/36718
- https://lab.llvm.org/buildbot/#/builders/259/builds/2039
- https://lab.llvm.org/buildbot/#/builders/36/builds/44091
- https://lab.llvm.org/buildbot/#/builders/272/builds/12629
- https://lab.llvm.org/buildbot/#/builders/271/builds/6020
- https://lab.llvm.org/buildbot/#/builders/236/builds/10319