Given an annotation element value that cannot be resolved (e.g. `@A(value = NoSuch.class)`), javac's implementation of AnnotationValue represents it using Attribute.UnresolvedClass.

UnresolvedClass implements AnnotationValue#accept as AnnotationValueVisitor#visitString for the string value `"<error>"`: http://hg.openjdk.java.net/jdk/jdk/file/807d192fb7dd/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Attribute.java#l399

Is this to spec? I am surprised it doesn't implement it using AnnotationValueVisitor#visitType with an javax.lang.model.type.ErrorType.

```
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.AnnotationMirror;
import javax.lang.model.element.AnnotationValue;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.lang.model.util.SimpleAnnotationValueVisitor8;

@interface A {
  Class<?> value() default Object.class;
}

@SupportedAnnotationTypes("*")
public class P extends AbstractProcessor {

  @Override
  public SourceVersion getSupportedSourceVersion() {
    return SourceVersion.latestSupported();
  }

  @Override
  public final boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    for (Element r : roundEnv.getRootElements()) {
      for (AnnotationMirror a : r.getAnnotationMirrors()) {
        System.err.println("annotation: " + a);
        for (AnnotationValue e : a.getElementValues().values()) {
          System.err.println("value: " + e);
          e.accept(
              new SimpleAnnotationValueVisitor8<Void, Void>() {
                @Override
                public Void visitString(String s, Void aVoid) {
                  System.err.println("string value: " + s);
                  return null;
                }
              },
              null);
        }
      }
    }
    return true;
  }
}
```

$ javac full version "14-ea+9-288"
annotation: @A(<error>)
value: <error>
string value: <error>
T.java:1: error: cannot find symbol
@A(value = B.class)
           ^
  symbol: class B
1 error