implement <rdar://problem/17279286> Swift has too many Unicode escape sequence forms

This consolidates the \x, \u, and \U escape sequences into one \u{abc} escape sequence.
For now we still parse and cleanly reject the old forms with a nice error message, this
will eventually be removed in a later beta (tracked by rdar://17527814)


Swift SVN r19435

stdlib/core/Unicode.swift

@@ -631,7 +631,7 @@ enum UTFDecodeResult {
       if stopOnError {
         return (hadError: true)
       }
-      OutputEncoding.encode("\ufffd", output: &output)
+      OutputEncoding.encode("\u{fffd}", output: &output)
       hadError = true
     }
   }