Ensure that 'auto' members of nested classes get parsed eagerly.

// For nested classes, auto member's must be parsed at their closing brace.
  // Typically - all delayed parsing of a nested class is delayed until the
  //   outermost class's closing brace - but we can not wait for a nested
  //   class with an auto member, since we must determine its layout at
  //   closing brace.
  // Since this routine can get called again when the outermost class is being
  // completed, we need to maintain idempotency by checking if the InitExpression
  // has already been parsed - and if so, do not reparse it.
  //  Note: the ActOnInitializerExpression is only called once, and that is
  //  at the completion of the outer most class.