Case build-in classes

Ref: http://php.net/manual/en/reserved.classes.php
In php allowed use any case of build-in class:

$exception = new Exception('Message');
$exception = new EXCEPTION('Message');
$exception = new eXcEpTiOn('Message');

Always return exception build-in class.
Should we prettify class names? Also don't forget about namespaces.

1. File without namespace - new Exception() === new \Exception();.
2. File with namespace - new Exception !== new \Exception.

In theory we can do it:

1. If namespace exists only correct new \Exception();
2. If namespace doesn't exists correct and new Exception() and new \Exception();

Start by reviewing the issue's PHP examples and the reserved-class and namespace rules it references. Done requires an agreed, namespace-aware decision for handling built-in class names in both namespaced and non-namespaced files, followed by implementation and coverage in the relevant formatter tests.