Fixed failed config schema loads due to libxml_disable_entity_loader
usage.

Applied CS patch.
bug #11259 [Config] Fixed failed config schema loads due to libxml_disable_entity_loader usage (ccorliss)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #11259).

Discussion
----------

[Config] Fixed failed config schema loads due to libxml_disable_entity_loader usage

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11258
| License       | MIT
| Doc PR        | N/A

PR #10493 helped this issue, but it can still affect users that:

1.  Have libxml_disable_entity_loader set to true by default.
2.  Experience libxml_disable_entity_loader php bug https://bugs.php.net/bug.php?id=64938

I used the same approach used in the DI xml validation.
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php#L452

Commits
-------

de2bef5 Fixed failed config schema loads due to libxml_disable_entity_loader usage.