Support enum field litral reference other fields

[egao1980]

/** Possible types of UCalendars 
 * @stable ICU 2.0
 */
enum UCalendarType {
  /**
   * Despite the name, UCAL_TRADITIONAL designates the locale's default calendar,
   * which may be the Gregorian calendar or some other calendar.
   * @stable ICU 2.0
   */
  UCAL_TRADITIONAL,
  /**
   * A better name for UCAL_TRADITIONAL.
   * @stable ICU 4.2
   */
  UCAL_DEFAULT = UCAL_TRADITIONAL,
  /**
   * Unambiguously designates the Gregorian calendar for the locale.
   * @stable ICU 2.0
   */
  UCAL_GREGORIAN
};

is currently transformed to:

(cffi:defcenum UCalendarType                                                                                        
    :UCAL_TRADITIONAL
    (:UCAL_DEFAULT #.UCAL_TRADITIONAL)
    :UCAL_GREGORIAN)