Attributes on non-ASTDecl types?

[Christoph Reichenbach]

My current understanding is that it is possible to attach attributes to classes, interfaces, and enums that are declared in jrag/ jadd files..
While I see the value wrt interfaces (despite bugs, cf. issue #340), it is not clear to me that synthesised attributes, collection attributes, inherited attributes, or NTAs make sense for classes and enums. I commented out the code generation for classes (didn’t find any for enums…), and all tests passed.

Since I couldn’t find a discussion of this capability in the reference manual, I assume that there is no intended use case for this capability.

Proposal:
- Adjust implementation so that it is an error to attach an attribute to a class or enum.