array_change_key_case() is not reported to return different case keys

[JeroenBakker]

Hi there, the title pretty much says it all, array_change_key_case() returns an array with the key cases changed, but phpstan complains that the keys aren't changed.

Reproduction: https://phpstan.org/r/ee95a2da-fe4c-44b6-8f06-369ecb271717
Proof that keys do change case: https://3v4l.org/TMciS

I'm working on a PR for phpstan/phpstan-src, but it might be a little while until I have the time to finish it.