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Issue Description:

Sorry for the undescriptive title of the issue

I have an iOS framework that requires extension-safe APIs (i.e. APPLICATION_EXTENSION_API_ONLY = YES)

In that framework I have a class defined as such:

@available(iOSApplicationExtension, unavailable)
public class UnavailableClass {
    init () { }
}

This compiles OK. But now I add another class that exposes this UnavailableClass as a property (the property is marked as unavailable as well)

class Container {
    @available(iOSApplicationExtension, unavailable)
    lazy var unavailable: UnavailableClass = UnavailableClass()
}

This does not compile with the Swift 5 compiler (neither on Swift 4.2.x mode nor on Swift 5 mode)

The error varies a bit depending how you declare that variable, and if the unavailable class comes from Swift or Objc, I've seen:

• 'Xxxx' is unavailable

• 'Xxxx' is unavailable: This class cannot be used from extension targets

I attach a sample project that demonstrates the issue, the project compiles fine on Xcode 10.1 (Swift 4.2.x), but fails to compile on Xcode 10.2 (both beta 1 and beta 2)

EDIT: Additional info: if I turn the property into a function, it does work.