Symfony version(s) affected: 5.1.9, 5.2.0 and possibly more

Description
When a config file in a symfony app (eg. config/services.yaml contains the following yaml, it will throw an exception:

    App\Example:
        arguments:
            $foo: 'bla\'

See the reproducer for more.

How to reproduce

https://github.com/Jeroeny/reproduce/tree/yaml

Originally thought to be related to #39229, but it's supposedly a different issue. The patch for that issue was also tested, but did not fix the bug.