[intl] Check for duplicate BCP 47 tags in a case-insensitive way

Intl constructors are specified to prohibit structurally invalid
subtags. BCP 47 defines itself to be case-insensitive. Firefox does
throw on case-insensitive duplicates, following the specifications.
This patch makes V8 do the same. There is some small compatibility
risk, but the case is fairly niche, so I hope it does not cause
much breakage.

BUG=v8:4215

Review-Url: https://codereview.chromium.org/2639333003
Cr-Commit-Position: refs/heads/master@{#42487}

src/js/i18n.js

@@ -851,6 +851,8 @@ function isStructuallyValidLanguageTag(locale) {
     return false;
   }
 
+  locale = %StringToLowerCase(locale);
+
   // Just return if it's a x- form. It's all private.
   if (%StringIndexOf(locale, 'x-', 0) === 0) {
     return true;

test/test262/local-tests/test/intl402/6.2.2_b.js

+// Copyright 2012 Mozilla Corporation. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+es5id: 6.2.2_b
+description: Tests that language tags with "_" are not accepted.
+author: Norbert Lindenberg
+includes: [testIntl.js]
+---*/
+
+var invalidLanguageTags = [
+    "de_DE",
+    "DE_de",
+    "cmn_Hans",
+    "cmn-hans_cn",
+    "es_419",
+    "es-419-u-nu-latn-cu_bob",
+    "i_klingon",
+    "cmn-hans-cn-t-ca-u-ca-x_t-u",
+    "enochian_enochian",
+    "de-gregory_u-ca-gregory",
+    "de-tester-Tester",  // Case-insensitive duplicate variant subtag
+    "de-DE-u-kn-true-U-kn-true",  // Case-insensitive duplicate singleton subtag
+];
+
+testWithIntlConstructors(function (Constructor) {
+    invalidLanguageTags.forEach(function (tag) {
+        var error;
+        try {
+            // this must throw an exception for an invalid language tag
+            var obj = new Constructor([tag]);
+        } catch (e) {
+            error = e;
+        }
+        if (error === undefined) {
+            $ERROR("Invalid language tag " + tag + " was not rejected.");
+        } else if (error.name !== "RangeError") {
+            $ERROR("Invalid language tag " + tag + " was rejected with wrong error " + error.name + ".");
+        }
+    });
+    return true;
+});