[turbofan] Enable typed lowering of string addition.

src/compiler/js-typed-lowering.h

Index: src/compiler/js-typed-lowering.h
diff --git a/src/compiler/js-typed-lowering.h b/src/compiler/js-typed-lowering.h
index 64931e1a0105774566749e0506e192ba2af25715..add45ce216a06da29f6419c04cfbe913d4c8e91b 100644
--- a/src/compiler/js-typed-lowering.h
+++ b/src/compiler/js-typed-lowering.h
@@ -72,6 +72,7 @@ class JSTypedLowering final : public AdvancedReducer {
   Factory* factory() const;
   Graph* graph() const;
   JSGraph* jsgraph() const { return jsgraph_; }
+  Isolate* isolate() const;
   JSOperatorBuilder* javascript() const;
   CommonOperatorBuilder* common() const;
   SimplifiedOperatorBuilder* simplified() { return &simplified_; }