subtypes of non-sealed types not handled correctly

Subtypes of non-sealed types incorrectly trigger the error message about needing one of sealed, non-sealed or final.

See the attached program. Try deleting 'final" from PublicImpl2a on line 8.

$ ./build/linux-x86_64-server-release/images/jdk/bin/javadoc -d play/private-sealed/api --enable-preview --source 15 play/private-sealed/src/C.java
Loading source file play/private-sealed/src/C.java...
Constructing Javadoc information...
play/private-sealed/src/C.java:8: error: one of: sealed, non-sealed or final modifiers expected
    public static class PublicImpl2a extends PublicImpl2 { }
                             ^
1 error

The message goes away when the class is made final.