[turbofan] Lower StringCharCodeAt to a dedicated builtin.

src/code-factory.h

Index: src/code-factory.h
diff --git a/src/code-factory.h b/src/code-factory.h
index 33c7faad05905e6d480bd6fdb0df04c8539d2546..16035dbfa249e908816d03b6f97e9e79c5cf8901 100644
--- a/src/code-factory.h
+++ b/src/code-factory.h
@@ -119,6 +119,7 @@ class V8_EXPORT_PRIVATE CodeFactory final {
   static Callable StringAdd(Isolate* isolate, StringAddFlags flags,
                             PretenureFlag pretenure_flag);
   static Callable StringCharAt(Isolate* isolate);
+  static Callable StringCharCodeAt(Isolate* isolate);
   static Callable StringCompare(Isolate* isolate, Token::Value token);
   static Callable StringEqual(Isolate* isolate);
   static Callable StringNotEqual(Isolate* isolate);