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

validation-test/stdlib/StringSlicesConcurrentAppend.swift

@@ -12,8 +12,8 @@ import Glibc
 var StringTestSuite = TestSuite("String")
 
 extension String {
-  var bufferID: UWord {
-    return unsafeBitCast(_core._owner, UWord.self)
+  var bufferID: UInt {
+    return unsafeBitCast(_core._owner, UInt.self)
   }
   var capacityInBytes: Int {
     return _core.nativeBuffer!.capacity