Don't use _getSuperclass() from the Swift runtime.

[grynspan]

To determine if a class is a subclass of another, we can use T.self is U.Type in an implicitly-opened existential context. Prior to the Swift 6 language mode, type existentials weren't openable, so we had to recursively call a runtime function to check this. That's no longer a concern for us.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test

[briancroom]

Cool!