Minimize AnnotationsImpl to leave it usable in simple scenarios where
there's no use-site targeted annotations, and use TargetedAnnotations in
the only place in the frontend (AnnotationResolverImpl) where use-sites
were needed.
Also, delete kdoc on Annotations.isEmpty to prevent readers from putting
too much thought into how it works. With the exception of a few places
in the frontend where use-site targeted annotations are still accessed
via the Annotations object, it's now an implementation detail and users
(such as backends, IDE) should not care about them at all, and instead
should just deal with the correct element when processing annotations
Alexander Udalov
eef11c669a