Suggest making it possible to retrieve the underlying JavaFileObject of a TypeElement

TypeElements usually have an underlying JavaFileObject, often either a .java or a .class one, but there is no public API to retrieve those JavaFileObjects.

This would be good for situations where a client compiles a source with JSR199, gets a Tree, and gets TypeElements out of that tree, and wants to know for instance if 1) the TypeElement comes from the original source, or 2) if the TypeElement comes from any JavaFileObject supplied by that client in their JavaFileManager implementation, OR if the client wants to access the underlying .java or .class file for other reasons, e.g. showing it in an editor.