static TypeSpec.Builder extensionClassTypeSpecBuilder(
      Context context, String className, String classToExtend, boolean isFinal) {
    return new ExtensionClassTypeSpecBuilder(context, className, classToExtend, isFinal)
        .extensionClassBuilder();
  }
A factory for {@link TypeSpec}s used in {@link AutoValueExtension} implementations.

<p>This is copied from {@link
com.google.auto.value.extension.memoized.processor.MemoizeExtension} until we find a better
location to consolidate the code.
value/src/main/java/com/google/auto/value/extension/toprettystring/processor/ExtensionClassTypeSpecBuilder.java