code: private static MethodRef getBuildMethod(Descriptor descriptor) {
      TypeInfo message = messageRuntimeType(descriptor);
      TypeInfo builder = builderRuntimeType(descriptor);
      return MethodRef.createInstanceMethod(
              builder, new Method("build", message.type(), NO_METHOD_ARGS))
          .asNonNullable();
    }
docs: Returns the {@link MethodRef} for the generated build method.