bug #20252 Trim constant values in XmlFileLoader (lstrojny)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #20252).

Discussion
----------

Trim constant values in XmlFileLoader

| Q             | A
| ------------- | ---
| Branch?       | 2.7, 2.8, 3.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n.A.
| License       | MIT
| Doc PR        | n.A.

When an XML config file gets long, it's nice to put the constant name into the next line like this:
```xml
<parameter key="som_very_very_long_constant_name" type="constant">
    Some\Namespace\That\Is\Sufficiently\Long\To\Require\A\LineBreak::someLongishConstantName
</parameter>
```

If that’s the case, `constant()` will try and find a constant including the spaces. While it is theoretically possible to have a constant in PHP that starts or ends with a space, it’s impossible for class constants and only doable using `define(" FOO ", …);`. So that’s probably an edge case we can ignore.

Commits
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f09e621 Trim constant values in XmlFileLoader

src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php

@@ -391,7 +391,7 @@ class XmlFileLoader extends FileLoader
                     $arguments[$key] = $arg->nodeValue;
                     break;
                 case 'constant':
-                    $arguments[$key] = constant($arg->nodeValue);
+                    $arguments[$key] = constant(trim($arg->nodeValue));
                     break;
                 default:
                     $arguments[$key] = XmlUtils::phpize($arg->nodeValue);