Fix cast error to ClassDescriptor

[vmishenev]

After #3053 we have a cast error in StdLib

[IgnatBeresnev]

Adding a test would be nice

[vmishenev]

We have the test source link should be for actual typealias already. To reproduce this issue, it needs another extensions order:

    val extensionsExtractor by extending {
        CoreExtensions.documentableTransformer with ExtensionExtractorTransformer() order {
            after(actualTypealiasAdder)
        }
    }

It is a general problem so I have opened #3125