DPrinter: improve display of impl-class, internal tag/kind, and external tag/kind

The AnnotatedType class displays the tag of underlying type, which makes it hard to quickly perceive the actual type.

A more general solution to always display simple name of impl class, internal tag/kind, and external tag/kind.

Note, for some JCTree nodes, no external kind is available; getKind() throws AssertionError.