Refactor formal parameter error locations into a class

src/parser.h

Index: src/parser.h
diff --git a/src/parser.h b/src/parser.h
index ee3727e165558508dca151c8b8e3225d9b9928a5..76fbac2a5643ada4b9a92b5b220843802d02eac4 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -755,8 +755,7 @@ class ParserTraits {
 
   // Utility functions
   int DeclareArrowParametersFromExpression(Expression* expression, Scope* scope,
-                                           Scanner::Location* undefined_loc,
-                                           Scanner::Location* dupe_loc,
+                                           FormalParameterErrorLocations* locs,
                                            bool* ok);
 
   // Temporary glue; these functions will move to ParserBase.