Issue title: Initializers declared as extension on Array are reported as unused
 Issue body: > Original issue: https://github.com/peripheryapp/issues/issues/3 by [ateliercw](/ateliercw)

Given a simplified example of:
```
import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        self.title = [String](title: "Title").first
    }
}

extension Array where Element == String {
    init(title: String) {
        self = [title]
    }
}
```
Periphery will report:
```
PeripheryTest/PeripheryTest/ViewController.swift:11:5: warning: Initializer 'init(title:)' is unused
```

This is causing a lot of false positives in a larger project that I'm working on do to expensive use of typed extensions on `Array` providing custom initializers.
 Comments: 
 Comment 0: @ateliercw This appears to be due to a SourceKit bug, it doesn't include any reference to the constructor when using an array literal. You can workaround it with `Array(title: "Title")`.

I'll need to submit a bug report to Apple for this. Not that I expect them to ever fix it, all my previous reports have had zero response 😢 
{'source': 'GithubIssues', 'repo_name_with_owner': 'peripheryapp/periphery', 'issue_id': 408561958, 'repo_stars': 3677.0}