private Optional<String> maybeForwardingClass(
      TypeElement autoBuilderType, Executable executable) {
    return executable.hasOptionalParameters()
        ? Optional.of(generatedClassName(autoBuilderType, "AutoBuilderBridge_"))
        : Optional.empty();
  }
Generates a class that will call the synthetic Kotlin constructor that is used to specify which
optional parameters are defaulted. Because it is synthetic, it can't be called from Java source
code. Instead, Java source code calls the {@code of} method in the class we generate here.
value/src/main/java/com/google/auto/value/processor/AutoBuilderProcessor.java