Migrate some case conversion functions from JS to CPP builtins

src/i18n.h

Index: src/i18n.h
diff --git a/src/i18n.h b/src/i18n.h
index f89d005b00c844309d925768f231c0bde94ca514..f1c160f6049607849ff4f432f7d949492f7e5e14 100644
--- a/src/i18n.h
+++ b/src/i18n.h
@@ -128,6 +128,16 @@ class V8BreakIterator {
   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