[stdlib] Add missing 10.10 NSString APIs.
[stdlib] Add missing 10.10 NSString APIs.

containsString and localizedCaseInsensitiveContainsString were
introduced in 10.10, release-noted, but never
documented (<rdar://22236574>), so we missed them.

Fixes <rdar://18776075> String.containsString doesn't work in Swift

Swift SVN r31152

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