Reland "Handlify concat string and substring."

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 5a2d5595fc9cf2fd12bf166494b22a0136db66d0..64ab2c058a985fb4f159c7f4c21a1b7562a1c384 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8647,11 +8647,6 @@ class String: public Name {
   // ASCII and two byte string types.
   bool MarkAsUndetectable();
 
-  // Return a substring.
-  MUST_USE_RESULT MaybeObject* SubString(int from,
-                                         int to,
-                                         PretenureFlag pretenure = NOT_TENURED);
-
   // String equality operations.
   inline bool Equals(String* other);
   bool IsUtf8EqualTo(Vector<const char> str, bool allow_prefix_match = false);