Symfony version(s) affected: 4.*

Description
Memcached adapter returns false on clear(). Does not actually do the cache clearing.

How to reproduce
Use Cache component, create some cache keys in Memcached and then try to do clear() on Memcached adapter. Always returns false.

Actual code of memcached trait:

protected function doClear($namespace)
{
    return false;
}

Possible Solution
Add $this->getClient()->flush(); on Memcached adapter trait.

Additional context