Move TypeAnnotationPosition from Attribute.Compound to Attribute.TypeCompound

The current implementation of type annotations models type annotations as a special case of annotations. However, the early stages of the javac pipeline have to consider that every annotation is potentially a type annotation. Therefore, the relationship should be reversed.

Under the present model, type annotations are represented with Attribute.TypeCompound, while regular annotations are represented with Attribute.Compound, and there are generally two separate codepaths for dealing with these two classes. This is a source of confusion and bugs, and should be eliminated.