Remove KVOContext from the Foundation overlay.
Remove KVOContext from the Foundation overlay.

We're recommending people just go with a global variable.

<rdar://problem/17272837>

Swift SVN r18837

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation