stdlib: stop using CFDictionaryCreateCopy() in dictionary bridging
stdlib: stop using CFDictionaryCreateCopy() in dictionary bridging

Use -[NSDictionary copyWithZone:] instead.

CFDictionaryCreateCopy() is buggy in OSes that ship today: it copies the
dictionary unconditionally, even if it is immutable, resulting in O(n)
bridging.

Swift SVN r27732

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