Symfony version(s) affected: 3.4 until master

Description
When wrapping an adapter inside a ProxyAdapter its impossible to use it within a TagAwareAdapter.

How to reproduce

$cache = new \Symfony\Component\Cache\Adapter\TagAwareAdapter(
    new \Symfony\Component\Cache\Adapter\ProxyAdapter(
        new \Symfony\Component\Cache\Adapter\ArrayAdapter()
    )
);

$item = $cache->getItem('foo');
$item->tag(['tag1', 'tag2']);
$item->set(true);
$cache->save($item);

var_dump($cache->getItem('foo')->get());

==>

array(2) {
  ["tag1"]=>
  int(0)
  ["tag2"]=>
  int(0)
}

So the actual value of the cache item is replaced by its tags.

@nicolas-grekas I tried to have a look and it seems to be related with the mysterious $innerItem of a CacheItem. If you could shed some light on this $innerItem and why it exists I might understand the logic and can fix it somehow 😊

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Cache/Adapter/TagAwareAdapter.php#L54