[Swift APIView] Decide how to handle protocol adoption

[tjprescott]

Related to: #2536

Here's a textbook example of protocol adoption from Apple's docs:

// Some public protocol
public protocol TextRepresentable {
    var textualDescription: String { get }
}

// Some struct that implements that protocol but doesn't declare it
public struct Hamster {
    public var name: String
    public var textualDescription: String {
        return "A hamster named \(name)"
    }
}

// Protocol adoption
public extension Hamster: TextRepresentable {}

APIView will completely ignore the extension because it doesn't contain anything public. However, this line is very important for the public API because it means that Hamster can be used anywhere TextRepresentable is accepted, but the APIView won't show that.

Option 1
Render the extension within the Hamster definition, even though this is not accurate Swift syntax.

public struct Hamster {
    public var name: String
    public var textualDescription: String {
        return "A hamster named \(name)"
    }
}

public extension Hamster: TextRepresentable {}

Option 2
Interpret the declaration and modify the struct definition, thus affecting how it is rendered:

// By interpreting the protocol adoption, we add it to the inheritance list here.
public struct Hamster: TextRepresentable {
    public var name: String
    public var textualDescription: String {
        return "A hamster named \(name)"
    }
}