Force Unwrapping rule doesn't apply to self

[skagedal]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Describe the bug

Linting the following snippet with force_unwrapping enabled in .swiftlint.yml:

class Test {
    func foo() {
        DispatchQueue.main.async { [weak self] in
            var string: String?
            _ = string!
            _ = self!
        }
    }
}

I would expect SwiftLint to warn on both force unwrappings. However, it does not catch self!.

Complete output when running SwiftLint, including the stack trace and command used

$ swiftlint        
Loading configuration from '.swiftlint.yml'
Linting Swift files at paths 
Linting 'foo.swift' (1/1)
/Users/simon/tmp/testswiftlint/foo.swift:5:23: warning: Force Unwrapping Violation: Force unwrapping should be avoided. (force_unwrapping)
Done linting! Found 1 violation, 0 serious in 1 file.

Environment

0.32.0 - installed with Homebrew

opt_in_rules:
  - force_unwrapping

Running Xcode 10.2.1 (confirmed with xcode-select -p), not using nested configurations.