[Macros] Allow reporting diagnostics on class keywords/identifiers

[rrousselGit]

Hello!

I'm currently experimenting with macros ; in particular error handling.
At the moment, we seem to have the ability to report warnings/errors on the class name, which is awesome. But in some cases, it may make more sense to show the warning in a slightly different location, such as on class keywords.

In particular, I'd like to show an error on the abstract keyword, because my macro requires a non-abstract classes.

Although I can currently show an error on the class name, it would be more developer friendly to show the error on the abstract instead