[Swift APIView] Generic types are not linkable when references.

When a generic type is declared, the generic elements are not registered as types themselves and thus references to them do not produce linkable behavior.

```swift
struct MyGeneric {
func doSomething() -> Element // in theory, this should be linkable to the definition in the declaration
}
```

This could get quite tricky though considering that many generics use common associated types (`Element` is very common) which could result in APIView getting confused.