[stdlib] Make _convertNSArrayToArray meet spec
[stdlib] Make _convertNSArrayToArray meet spec

I believe this correctly implements the "forced conversion" part of the
Arrays.rst document.

Swift SVN r18248

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