code: AnnotationInfo getImmutableAnnotation(Symbol sym, VisitorState state) {
      String nameStr = sym.flatName().toString();
      AnnotationInfo known = wellKnownMutability.getKnownImmutableClasses().get(nameStr);
      if (known != null) {
        return known;
      }
      return threadSafety.getInheritedAnnotation(sym, state);
    }
docs: Gets the {@link Symbol}'s {@code @Immutable} annotation info, either from an annotation on the
  symbol or from the list of well-known immutable types.