[csa] Migrate String.prototype.concat to TurboFan builtin.

src/code-factory.h

Index: src/code-factory.h
diff --git a/src/code-factory.h b/src/code-factory.h
index 73550441165770c24eb78bbeb628b19bac7b824c..62e860f42a3f15fa08a3d7376d588b1813466085 100644
--- a/src/code-factory.h
+++ b/src/code-factory.h
@@ -108,8 +108,9 @@ class V8_EXPORT_PRIVATE CodeFactory final {
   static Callable Equal(Isolate* isolate);
   static Callable StrictEqual(Isolate* isolate);
 
-  static Callable StringAdd(Isolate* isolate, StringAddFlags flags,
-                            PretenureFlag pretenure_flag);
+  static Callable StringAdd(Isolate* isolate,
+                            StringAddFlags flags = STRING_ADD_CHECK_NONE,
+                            PretenureFlag pretenure_flag = NOT_TENURED);
   static Callable StringCharAt(Isolate* isolate);
   static Callable StringCharCodeAt(Isolate* isolate);
   static Callable StringCompare(Isolate* isolate, Token::Value token);