Fix crash under lb_LU locale

Bug: v8:9642
Change-Id: I2dcd1c0e3c208b15b5c0ec0f08880744134f7474
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1769479Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63404}

src/objects/js-date-time-format.cc

@@ -8,8 +8,11 @@
 
 #include "src/objects/js-date-time-format.h"
 
+#include <algorithm>
+#include <map>
 #include <memory>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "src/date/date.h"
@@ -1191,6 +1194,12 @@ class DateTimePatternGeneratorCache {
     UErrorCode status = U_ZERO_ERROR;
     map_[key].reset(icu::DateTimePatternGenerator::createInstance(
         icu::Locale(key.c_str()), status));
+    // Fallback to use "root".
+    if (U_FAILURE(status)) {
+      status = U_ZERO_ERROR;
+      map_[key].reset(
+          icu::DateTimePatternGenerator::createInstance("root", status));
+    }
     CHECK(U_SUCCESS(status));
     return map_[key]->clone();
   }

test/intl/regress-9642.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Environment Variables: LANG=lb_LU
+// Test creation of Intl.DateTimeFormat under not installed locale.
+
+let dtf = new Intl.DateTimeFormat();