The initial solution to gh-11512 was still using a plain `toString` that
could potentially break with a JDK upgrade. Turns out that JDK9 actually
uses the same type for AnnotatedType and ClassType so the trick of using
a visitor doesn't work anymore.

Retrospectively, it is quite easy to generate the full type once we have
the DeclaredType as we already have some logic to get the qualified,
that is raw, type and we have access to the type parameters.

This commit still uses a `toString` to generate the representation of
the type parameters but this looks much safer than trying to redo what
such a simple `toString` should do. Also, the additional metadata that
we could get on an ExecutableElement does not apply to them.

Closes gh-11512