Suppress KVO warning when the property has an explicit setter

[theblixguy]

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.

Resolves SR-12286
Resolves rdar://problem/59702530
Resolves FB7595420

[theblixguy]

@swift-ci please smoke test

[xedin]

@swift-ci please smoke test

[theblixguy]

@xedin I'll create a new PR against swift-5.2 branch :)

[theblixguy]

Unrelated failure:

/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/branch-master/swiftpm/Tests/FunctionalTests/DependencyResolutionTests.swift:37: error: DependencyResolutionTests.testInternalComplex : failed - `swift build -c Release' failed:

@swift-ci please smoke test Linux