Reland "[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 3e6a446ec7ca9a3e5202279a9a23b6aa3f78a9bc..936c5d267799b52aee590fc4cbe85ae5e9143a57 100644
--- a/src/code-stub-assembler.h
+++ b/src/code-stub-assembler.h
@@ -19,6 +19,12 @@ class StubCache;
 
 enum class PrimitiveType { kBoolean, kNumber, kString, kSymbol };
 
+enum class UnicodeEncoding {
+  // Different unicode encodings in a |word32|:
+  UTF16,  // hi 16bits -> trailing surrogate or 0, low 16bits -> lead surrogate
+  UTF32,  // full UTF32 code unit / Unicode codepoint
+};
+
 // Provides JavaScript-specific "macro-assembler" functionality on top of the
 // CodeAssembler. By factoring the JavaScript-isms out of the CodeAssembler,
 // it's possible to add JavaScript-specific useful CodeAssembler "macros"
@@ -472,6 +478,9 @@ class CodeStubAssembler : public compiler::CodeAssembler {
   compiler::Node* SubString(compiler::Node* context, compiler::Node* string,
                             compiler::Node* from, compiler::Node* to);
 
+  compiler::Node* StringFromCodePoint(compiler::Node* codepoint,
+                                      UnicodeEncoding encoding);
+
   // Type conversion helpers.
   // Convert a String to a Number.
   compiler::Node* StringToNumber(compiler::Node* context,