Refine duplicate arrow function parameter location handling

This patch fixes the logic of finding the location of a duplicate
parameter error in arrow functions by only looking at the error if it
exists. This should address a TSAN error. Further, an UNREACHABLE()
statement is inserted somewhere in the ExpressionClassifier code
to make future similar bugs easier to find.

BUG=v8:4483

Review-Url: https://codereview.chromium.org/2365693004
Cr-Commit-Position: refs/heads/master@{#39701}

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;
   }

src/parsing/parser.cc

@@ -2609,7 +2609,9 @@ void Parser::ParseArrowFunctionFormalParameterList(
   for (int i = 0; i < parameters->Arity(); ++i) {
     auto parameter = parameters->at(i);
     DeclareFormalParameter(parameters->scope, parameter);
-    if (!duplicate_loc->IsValid()) {
+    if (!this->classifier()
+             ->is_valid_formal_parameter_list_without_duplicates() &&
+        !duplicate_loc->IsValid()) {
       *duplicate_loc =
           this->classifier()->duplicate_formal_parameter_error().location;
     }