Issue Description:

Summary:
When a Swift class contains an optional Enum property that is marked with @objc, and this same class also contains a mutable (var) array holding at least one element, there is an unexpected memory leak.

Steps to Reproduce:
Steps:

1. Download the attachment project (EnumMemoryLeak.zip) OR clone if from: https://github.com/Giphy/ios-memory-leak-sample
2. Open the project in XCode
3. Command + I
4. Select Leaks Instrument
5. Record

You should now encounter leaks.

Expected Results:
No leaks.

Observed Results:
Memory leaking.

Version:
XCode:

• Version 9.0 beta 4 (9M189t)
  (Same issue on 8.3.3 as well)

Deployment Target:

• 8.0+

MacOS:

• Version 10.12.5 (16F73)

Notes:
Work around:

If we convert the optional enum class property to a non-optional, leak will disappear.

// Let's convert the optional property to a non-optional
var leakOptionalEnum: leakingObjCMarkedEnum = .memoryLeakCase