final void checkModifiersIfNested(TypeElement type) {
    checkModifiersIfNested(type, type, simpleAnnotationName);
  }
Checks that, if the given {@code @AutoValue}, {@code @AutoOneOf}, or {@code @AutoBuilder} class
is nested, it is static and not private. This check is not necessary for correctness, since the
generated code would not compile if the check fails, but it produces better error messages for
the user.
value/src/main/java/com/google/auto/value/processor/AutoValueishProcessor.java