stdlib: remove Word and UWord

These types are leftovers from the early pre-1.0 times when Int and UInt
were always 64-bit on all platforms.  They serve no useful purpose
today.  Int and UInt are defined to be word-sized and should be used
instead.

rdar://18693488

Swift SVN r30564

stdlib/public/SDK/ObjectiveC/ObjectiveC.swift

@@ -254,7 +254,7 @@ public func == (lhs: NSObject, rhs: NSObject) -> Bool {
 extension NSObject : CVarArgType {
   /// Transform `self` into a series of machine words that can be
   /// appropriately interpreted by C varargs
-  public var _cVarArgEncoding: [Word] {
+  public var _cVarArgEncoding: [Int] {
     _autorelease(self)
     return _encodeBitsAsWords(self)
   }