Match expression does not handle remaining value: class-string

# Bug report

PHPStan does not recognise that all match arms are handled when you use a class union type.

### Code snippet that reproduces the problem

https://phpstan.org/r/51d5cd6f-2479-4db6-ae2b-cfae97da918d

### Expected output

Since all match arms are covered, no error should happen.

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