bug symfony#25380 [FrameworkBundle][Cache] register system cache clearer only if it's used (xabbuh)

This PR was merged into the 3.4 branch.

Discussion
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[FrameworkBundle][Cache] register system cache clearer only if it's used

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#25326
| License       | MIT
| Doc PR        |

Commits
-------

093eb3d register system cache clearer only if it's used

Author: fabpot

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Compiler/CachePoolPass.php

@@ -112,6 +112,10 @@ public function process(ContainerBuilder $container)
                 $clearer->setArgument(0, $pools);
             }
             $clearer->addTag('cache.pool.clearer');
+
+            if ('cache.system_clearer' === $id) {
+                $clearer->addTag('kernel.cache_clearer');
+            }
         }
     }
 

src/Symfony/Bundle/FrameworkBundle/Resources/config/cache.xml

@@ -104,9 +104,7 @@
             <argument type="collection" />
         </service>
 
-        <service id="cache.system_clearer" parent="cache.default_clearer" public="true">
-            <tag name="kernel.cache_clearer" />
-        </service>
+        <service id="cache.system_clearer" parent="cache.default_clearer" public="true" />
 
         <service id="cache.global_clearer" parent="cache.default_clearer" public="true" />
         <service id="cache.app_clearer" alias="cache.default_clearer" public="true" />