bug #14900 Silence deprecation warnings by default (reecefowell)

This PR was merged into the 2.7 branch.

Discussion
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[RFC] Fixing Deprecation Warnings

This PR address an issue that causes Symfony to vomit E_DEPRECATED warnings everywhere.

```markdown
| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #14899
| License       | MIT
```

Fixes #14899

![http://cdn.meme.am/instances2/500x/125359.jpg](http://cdn.meme.am/instances2/500x/125359.jpg)

Commits
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73bbaa6 Silence invasive deprecation warnings, opt-in for warnings

Author: nicolas-grekas

ConfigCache.php

@@ -46,7 +46,7 @@ public function __construct($file, $debug)
      */
     public function __toString()
     {
-        trigger_error('ConfigCache::__toString() is deprecated since version 2.7 and will be removed in 3.0. Use the getPath() method instead.', E_USER_DEPRECATED);
+        @trigger_error('ConfigCache::__toString() is deprecated since version 2.7 and will be removed in 3.0. Use the getPath() method instead.', E_USER_DEPRECATED);
 
         return $this->file;
     }

Definition/ReferenceDumper.php

@@ -11,7 +11,7 @@
 
 namespace Symfony\Component\Config\Definition;
 
-trigger_error('The '.__NAMESPACE__.'\ReferenceDumper class is deprecated since version 2.4 and will be removed in 3.0. Use the Symfony\Component\Config\Definition\Dumper\YamlReferenceDumper class instead.', E_USER_DEPRECATED);
+@trigger_error('The '.__NAMESPACE__.'\ReferenceDumper class is deprecated since version 2.4 and will be removed in 3.0. Use the Symfony\Component\Config\Definition\Dumper\YamlReferenceDumper class instead.', E_USER_DEPRECATED);
 
 use Symfony\Component\Config\Definition\Dumper\YamlReferenceDumper;