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

test/1_stdlib/NSStringAPI.swift

@@ -1765,7 +1765,7 @@ struct ComparisonTest {
   init(
     _ expectedUnicodeCollation: ExpectedComparisonResult,
     _ lhs: String, _ rhs: String,
-    file: String = __FILE__, line: UWord = __LINE__
+    file: String = __FILE__, line: UInt = __LINE__
   ) {
     self.expectedUnicodeCollation = expectedUnicodeCollation
     self.lhs = lhs