DX: Supply CacheBackendInterface::CACHE_PERMANENT as Cache::PERMANENT

Full conversion of all instances + use statements throughout core, courtesy of @dawehner + phpStorm ;)

For now, I'm intentionally leaving the BC-shim in place; i.e.:

class Cache {

  const PERMANENT = CacheBackendInterface::CACHE_PERMANENT;
}

Doing so should hopefully break less patches in the queue right now.

And perhaps we even want to retain that as-is (not marking CacheBackendInterface::CACHE_PERMANENT as @deprecated). Technically, the constant/value is not really owned by CacheBackend, but would rather belong to a (non-existing) CacheItem class, since it is the cache item that declares when it wants to expire.

Since there is no CacheItem class, and also, given our current efforts that are seemingly moving all of the remainders of cache.inc into the Cache class, the Cache class appears to be the most appropriate home for now.

1: drupal8.cache-dx.1.patch queued for re-testing.

Back to green.

Quoting myself:

Technically, the constant/value is not really owned by CacheBackend, but would rather belong to a (non-existing) CacheItem class, since it is the cache item that declares when it wants to expire.

Since there is no CacheItem class, and also, given our current efforts that are seemingly moving all of the remainders of cache.inc into the Cache class, the Cache class appears to be the most appropriate home for now.

Please note that work on a CacheItem class + interface happens in #1748022: Make CacheBackendInterface::get() return a proper class

That said, even with that, the DX of Cache::PERMANENT would still be much nicer than CacheItemInterface::PERMANENT.