[stdlib] _SwiftNativeNS*.init(): Add @nonobjc attribute

[lorentey]

This fixes an @objc inference warning for _SwiftNativeNSArray.init during stdlib builds.

[lorentey]

@swift-ci please smoke test

[jrose-apple]

You probably want this to be @nonobjc, though, because there's no reason to expose this to Objective-C. Right?

[lorentey]

Hm, I chose @objc because it is overriding NSArray.init() in an underhanded way -- but it shouldn't ever be called from Objective-C. Do you think it's safe to flip it to @nonobjc?

If so, we can also change some additional cases below this one.

[jrose-apple]

Do you get a valid _SwiftNativeNSArray out if [[_SwiftNativeNSArray alloc] init] actually gets called? If not, I don't think it's interesting.

[lorentey]

It all works out; alloc gets the SwiftObject implementation, and init is empty anyway. 🎉
I switched _SwiftNativeNSArray.init and three similar initializers to @nonobjc.

[lorentey]

@swift-ci please smoke test

[lancep]

LGTM!