Issue Description:

Given the following code:

extension String.StringInterpolation {
    mutating func appendInterpolation(test value: Int) {
        appendInterpolation(value)
    }
}

public struct Fixture112 {
    public func someFunc() {
        print("test: \(test: 1)")
    }
}

The index store could include a relation from the someFunc() occurrence to appendInterpolation(test:). For the purposes of identifying unused code, it's not currently possible to identify that appendInterpolation(test:) is used in the string interpolation.