[Parse] Upgrade unlabeled_parameter_following_variadic_parameter to Swift 4 error.

Part of rdar://problem/28961650 .

Author: huonw

include/swift/AST/DiagnosticsParse.def

@@ -851,8 +851,10 @@ ERROR(parameter_curry_syntax_removed,none,
 ERROR(initializer_as_typed_pattern,none,
       "unexpected initializer in pattern; did you mean to use '='?", ())
 
-WARNING(unlabeled_parameter_following_variadic_parameter,none,
+WARNING(swift3_unlabeled_parameter_following_variadic_parameter,none,
         "a parameter following a variadic parameter requires a label", ())
+ERROR(unlabeled_parameter_following_variadic_parameter,none,
+      "a parameter following a variadic parameter requires a label", ())
 
 //------------------------------------------------------------------------------
 // Statement parsing diagnostics

lib/Parse/ParsePattern.cpp

@@ -439,8 +439,11 @@ mapParsedParameters(Parser &parser,
     // Warn when an unlabeled parameter follows a variadic parameter
     if (ellipsisLoc.isValid() && elements.back()->isVariadic() &&
         param.FirstName.empty()) {
-      parser.diagnose(param.FirstNameLoc,
-                      diag::unlabeled_parameter_following_variadic_parameter);
+      auto message =
+          parser.Context.isSwiftVersion3()
+              ? diag::swift3_unlabeled_parameter_following_variadic_parameter
+              : diag::unlabeled_parameter_following_variadic_parameter;
+      parser.diagnose(param.FirstNameLoc, message);
     }
 
     // If this parameter had an ellipsis, check whether it's the last parameter.