public static Optional<AnnotationSpec> generatedAnnotationSpec(
      Elements elements, SourceVersion sourceVersion, Class<?> processorClass) {
    return generatedAnnotationSpecBuilder(elements, sourceVersion, processorClass)
        .map(AnnotationSpec.Builder::build);
  }
Returns {@code @Generated("processorClass")} for the target {@code SourceVersion}.

<p>Returns {@code javax.annotation.processing.Generated} for JDK 9 and newer, {@code
javax.annotation.Generated} for earlier releases, and Optional#empty()} if the annotation is
not available.
common/src/main/java/com/google/auto/common/GeneratedAnnotationSpecs.java