[SE-0069] Added Null value type for Darwin Foundation

[colemancda]

What's in this pull request?

Adds a struct Null to the Swift 3.0 Foundation value types. As a struct, Null is more performant (no ARC or memory allocation) than NSNull, and will be needed for Swift JSON decoders and libraries that want to use struct value types, and be free from classes for their model layer.

Resolved bug number: (SR-1880)

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[lattner]

This is a library extension that needs to go through the evolution process.