Fix docblock and typehint stdClass for MemcacheBackend::valid

  }

  /**

   * {@inheritdoc}

   * Determines if the cache item is valid.

   *

   * This also alters the valid property of the cache item itself.

   *

   * @param string $cid

   *   The cache ID.

   * @param \stdClass $cache

   *   The cache item.

   *

   * @return bool

   */

  protected function valid($cid, $cache) {

  protected function valid($cid, \stdClass $cache) {

    $lock_key = "memcache_$cid:$this->bin";

    $cache->valid = FALSE;