[5.2] Suppress KVO warning when the property has an explicit setter

[theblixguy]

• Explanation:

In #28135, we added a new warning when a non-@objc dynamic property is passed to the KVO observe method, because passing such a property can lead to unexpected behaviour or runtime trap.

However, the user might have an @objc property (without the dynamic modifier) and perform the updates manually, by calling willChangeValue and didChangeValue:

private var _bar: Int = 0
@objc var bar: Int {
    get { _bar }
    set {
     guard newValue != _bar else { return }
     willChangeValue(for: \.bar)
     _bar = newValue
     didChangeValue(for: \.bar)
    }
  }

This is okay to do, so suppress the warning if we have an explicit setter on the @objc property. We could go further and check for those specific calls, but it's probably safe to assume that those calls are being made if there's an explicit setter.

• Issue: SR-12286 | rdar://problem/59702530 | FB7595420

• Scope: Affects use of @objc properties with manual KVO events when passed to the KVO observe method.

• Risk: Very Low. We're just suppressing a warning diagnostic, which shouldn't appear in the above scenario.

• Testing: Added a new test case.

• Reviewed by: @xedin

Cherry pick of #30098

Resolves: rdar://problem/59702530

[xedin]

@swift-ci please test