Move nil/_Nil into the core library and add UnsafePointer<T> support.

Fixes <rdar://problem/15933510>.


Swift SVN r13094

stdlib/core/Nil.swift

@@ -0,0 +1,12 @@
+@asmname="swift_makeUnsafeNil" func _makeUnsafeNil() -> AnyObject
+
+struct _Nil {
+  @conversion func __conversion() -> AnyObject {
+    return _makeUnsafeNil()
+  }
+}
+
+/// A null pointer constant.
+var nil : _Nil {
+  return _Nil()
+}