[builtins] migrate C++ String Iterator builtins to baseline TurboFan

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index fee4ab36cce480e8d4bec0455ab318c488c7d08c..b467ac607fd9907c579f1bcd1bbae0c0d742d13c 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -785,7 +785,8 @@ void JSWeakMap::JSWeakMapVerify() {
 void JSStringIterator::JSStringIteratorVerify() {
   CHECK(IsJSStringIterator());
   JSObjectVerify();
-  CHECK(string()->IsString());
+  CHECK(string()->IsSeqString() || string()->IsExternalString());
+
   CHECK_GE(index(), 0);
   CHECK_LE(index(), String::kMaxLength);
 }