[Swift APIView] APIView missing type inheritance that appears in code

We have this example from Apple's documentation:

```Swift
extension Array: TextRepresentable where Element: TextRepresentable {
public var textualDescription: String {
let itemsAsText = self.map { $0.textualDescription }
return "[" + itemsAsText.joined(separator: ", ") + "]"
}
}
```

This renders in APIView as:
```Swift
extension Array where Element: TextRepresentable {
public var textualDescription: String
}
```

The curious thing is that the `TextRepresentable` annotation on `Array` is removed. Looking into the AST, it appears this information is `nil` in the node, suggesting that perhaps it isn't relevant or the compiler deems it redundant. The other alternative is a bug in the AST library.

Start by reproducing the Apple documentation example in Swift APIView and inspect the AST node for the extension's type inheritance, where the TextRepresentable annotation appears to be nil. Compare the rendered output with the source; done when the Array inheritance annotation is preserved in the APIView output.

Written by the indexing model from the issue text.