static Optional<AnnotationMirror> getAnnotationMirror(Element element, String annotationName) {
    for (AnnotationMirror annotation : element.getAnnotationMirrors()) {
      TypeElement annotationElement = MoreTypes.asTypeElement(annotation.getAnnotationType());
      if (annotationElement.getQualifiedName().contentEquals(annotationName)) {
        return Optional.of(annotation);
      }
    }
    return Optional.empty();
  }
Returns the {@code @AutoValue} or {@code @AutoOneOf} type parameters, with a ? for every type.
If we have {@code @AutoValue abstract class Foo<T extends Something>} then this method will
return just {@code <?>}.
value/src/main/java/com/google/auto/value/processor/AutoValueishProcessor.java