Iterable of immediate "parents" to try resolving component parts. referenceBy stops at the first parent where a part is found. Can be cached.
// TODO(jcollins-g): Rationalize the different "enclosing" types so that // this doesn't duplicate `[enclosingElement]` in many cases. // TODO(jcollins-g): Implement comment reference resolution via categories, // making the iterable make sense here. Iterable<CommentReferable> get referenceParents;