Mixed contains everything

[orklah]

This should fix #5088.

However, I'm not entirely sure this is the right fix.

The ternary here seems to handle two cases:

• we're in PHP <= 7.3 or we don't have a signature type, then we go in UnionTypeComparator::isContainedByInPhp
• we're in PHP >= 7.4 and we have a signature type, then we go in UnionTypeComparator::isContainedBy

The case in the test is pretty in between because we have a signature type only in the implementation. I don't really know why there are two methods for handling signature and phpdoc, by I kinda have the feeling we enter the wrong one.

[muglug]

I think it's good!