target annotation for object annotations

[scabug]

for top-level object definitions the scala compiler generates two classes (two classfiles), on named "ObjectName$$" (containing the actual implementation) and a "mirror class" named "ObjectName" wich contains static methods that forward to the actual implementations. The second one is generated for Java compatibility only, to ease accessing object members from java.

The scala compiler copies all annotations from the object to the mirror class.

We should just make this configurable using the target-meta-annotations.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-3599?orig=1
Reporter: @lrytz
See #3115