Make pointer nullability explicit using Optional.

Implements SE-0055: https://github.com/apple/swift-evolution/blob/master/proposals/0055-optional-unsafe-pointers.md

- Add NULL as an extra inhabitant of Builtin.RawPointer (currently
  hardcoded to 0 rather than being target-dependent).
- Import non-object pointers as Optional/IUO when nullable/null_unspecified
  (like everything else).
- Change the type checker's *-to-pointer conversions to handle a layer of
  optional.
- Use 'AutoreleasingUnsafeMutablePointer<NSError?>?' as the type of error
  parameters exported to Objective-C.
- Drop NilLiteralConvertible conformance for all pointer types.
- Update the standard library and then all the tests.

I've decided to leave this commit only updating existing tests; any new
tests will come in the following commits. (That may mean some additional
implementation work to follow.)

The other major piece that's missing here is migration. I'm hoping we get
a lot of that with Swift 1.1's work for optional object references, but
I still need to investigate.

test/1_stdlib/NSStringAPI.swift

@@ -33,7 +33,7 @@ class NonContiguousNSString : NSString {
     super.init()
   }
 
-  @objc(copyWithZone:) override func copy(with zone: NSZone) -> AnyObject {
+  @objc(copyWithZone:) override func copy(with zone: NSZone?) -> AnyObject {
     // Ensure that copying this string produces a class that CoreFoundation
     // does not know about.
     return self