New Issue Checklist

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

Describe the bug

Swiftlint tells me that I need to add an access control level for an extension that adds protocol conformance. However, the Swift compiler won't allow me to do that -- when I try to build I get
error: 'public' modifier cannot be used with extensions that declare protocol conformances. (Note I get an equivalent error if I use any other ACL, private, internal, etc.)

This can be reproduced in a directory containing just the following two files:

lint.swift

public struct MyStruct {}
extension MyStruct: Equatable {}

.swiftlint.yml

opt_in_rules:
  - explicit_acl

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

kaitlinmahar@kaitlin:~/lint-test$ swiftlint lint
Loading configuration from '.swiftlint.yml'
Linting Swift files at paths
Linting 'lint.swift' (1/1)
/Users/kaitlinmahar/lint-test/lint.swift:2:1: warning: Explicit ACL Violation: All declarations should specify Access Control Level keywords explicitly. (explicit_acl)
Done linting! Found 1 violation, 0 serious in 1 file.

If I change the file to:

public struct MyStruct {}
public extension MyStruct: Equatable {}

Swiftlint is satisfied --

kaitlinmahar@kaitlin:~/lint-test$ swiftlint lint
Loading configuration from '.swiftlint.yml'
Linting Swift files at paths
Linting 'lint.swift' (1/1)
Done linting! Found 0 violations, 0 serious in 1 file.

But the compiler is not:

kaitlinmahar@kaitlin:~/lint-test$ swift lint.swift
lint.swift:2:1: error: 'public' modifier cannot be used with extensions that declare protocol conformances
public extension MyStruct: Equatable {}
^~~~~~~

Environment

• SwiftLint version (run swiftlint version to be sure)? 0.31.0
• Installation method used (Homebrew, CocoaPods, building from source, etc)? Homebrew
• Which Xcode version are you using (check xcode-select -p)? xcode-select version 2354. However, I'm using Swift 4.2 installed via swiftenv.

kaitlinmahar@kaitlin:~/lint-test$ swift --version
Apple Swift version 4.2 (swift-4.2-RELEASE)
Target: x86_64-apple-darwin18.2.0

I also switched to Swift 5.0 via swiftenv and tested, same results.