argument
(https://github.com/ruby/rdoc/pull/1222)

It is necessary for ClassModule's instance variable @superclass to
always be a String (or nil) so that the class can be saved with
`#marshal_dump` and loaded with `#marshal_load`.

However, there's no type checking being done, which allows a bug like
the one reported in #1221 (which was introduced in #1217) that sets
superclass to a ClassModule. That bug requires:

- setting a superclass to a NormalClass
- marshal_save
- marshal_load (which raises an exception)

With this change, passing a ClassModule to ClassModule#superclass= is
explicitly allowed by saving the full name of the ClassModule in the
@superclass instance variable.

https://github.com/ruby/rdoc/commit/9ced6d534c