Summary

Ensure modifiable with primitive field and default value returns the default value
and .*IsSet() returns false after .unset*() is called.

Description:

Currently a field such as:

@Value.Modifiable
interface Standalone {

  @Value.Default
  default int def() {
    return 1;
  }
}

Would generate the following for the .unset*() method:
e.g.

/**
 * Reset an attribute to its initial value.
 * @return {@code this} for use in a chained invocation
 */
@CanIgnoreReturnValue
public final ModifiableStandalone unsetDef() {
  optBits |= 0;
  def = 0;
  return this;
}

Having optBits |= 0; does nothing, and doesn't effectively unset the field. Thus, subsequent calls to .def() would return 0, instead of the value returned by the method annotated with @Value.Default (i.e. 1). Further, subsequent calls to .defIsSet() would return true when it should return false.

This change would instead generate the following for the .unset*() method:
e.g.

/**
 * Reset an attribute to its initial value.
 * @return {@code this} for use in a chained invocation
 */
@CanIgnoreReturnValue
public final ModifiableStandalone unsetDef() {
  optBits &= ~OPT_BIT_DEF;
  def = 0;
  return this;
}

Having optBits &= ~OPT_BIT_DEF will cause subsequent calls to .defIsSet() to return false, and calls to .def() to invoke the method annotated with @Value.Default to get the value.