Migrate some case conversion functions from JS to CPP builtins

src/i18n.h

 // Copyright 2013 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.
 // limitations under the License.
 
 #ifndef V8_I18N_H_
 #define V8_I18N_H_
 
 #include "src/objects.h"
 #include "unicode/uversion.h"
@@ skipping 110 matching lines @@
 
   // Layout description.
   static const int kBreakIterator = JSObject::kHeaderSize;
   static const int kUnicodeString = kBreakIterator + kPointerSize;
   static const int kSize = kUnicodeString + kPointerSize;
 
  private:
   V8BreakIterator();
 };
 
+const UChar* GetUCharBufferFromFlat(const String::FlatContent& flat,
+                                    std::unique_ptr<uc16[]>* dest,
+                                    int32_t length);
+MUST_USE_RESULT Object* LocaleConvertCase(Handle<String> s, Isolate* isolate,
+                                          bool is_to_upper, const char* lang);
+MUST_USE_RESULT Object* ConvertToLower(Handle<String> s, Isolate* isolate);
+MUST_USE_RESULT Object* ConvertToUpper(Handle<String> s, Isolate* isolate);
+MUST_USE_RESULT Object* ConvertCase(Handle<String> s, bool is_upper,
+                                    Isolate* isolate);
+
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_I18N_H_