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

src/code-stub-assembler.h

Index: src/code-stub-assembler.h
diff --git a/src/code-stub-assembler.h b/src/code-stub-assembler.h
index 76bd5dee425d1884140b46c0997fff4abdceadec..dc6f0637a00471d66caebb5732b78d2ef6ffaf25 100644
--- a/src/code-stub-assembler.h
+++ b/src/code-stub-assembler.h
@@ -444,6 +444,11 @@ class CodeStubAssembler : public compiler::CodeAssembler {
   // Return the single character string with only {code}.
   compiler::Node* StringFromCharCode(compiler::Node* code);
 
+  // Return a String made up of the codepoint at index.
+  compiler::Node* StringFromCodePointAt(compiler::Node* string,
+                                        compiler::Node* length,
+                                        compiler::Node* index);
+
   // Type conversion helpers.
   // Convert a String to a Number.
   compiler::Node* StringToNumber(compiler::Node* context,