Update validation testsuite for the argument label default changes.

Swift SVN r27706

validation-test/stdlib/UnicodeUTFEncoders.swift

@@ -68,9 +68,9 @@ func nthUnicodeScalar(n: UInt32) -> UnicodeScalar {
 // `buffer` should have a length >= 4
 func nsEncode<CodeUnit>(
   var c: UInt32,
-  encoding: NSStringEncoding,
-  inout buffer: [CodeUnit],
-  inout used: Int
+  _ encoding: NSStringEncoding,
+  inout _ buffer: [CodeUnit],
+  inout _ used: Int
 ) {
   _precondition(buffer.count >= 4, "buffer is not large enough")