Refine duplicate arrow function parameter location handling

src/parsing/expression-classifier.h

Index: src/parsing/expression-classifier.h
diff --git a/src/parsing/expression-classifier.h b/src/parsing/expression-classifier.h
index bc308dd1883ed0dccc32b4e529f2dbd9f783fb5d..6a1fbac35a0c108b0da61473b42dca36a4f29ed5 100644
--- a/src/parsing/expression-classifier.h
+++ b/src/parsing/expression-classifier.h
@@ -410,6 +410,9 @@ class ExpressionClassifier {
     // We should only be looking for an error when we know that one has
     // been reported.  But we're not...  So this is to make sure we have
     // the same behaviour.
+    UNREACHABLE();
+
+    // Make MSVC happy by returning an error from this inaccessible path.
     static Error none;
     return none;
   }