[examples][BuildingAJIT] Use the right layer when adding code in Chapter 3.

We were incorrectly using the OptimizeLayer and bypassing the COD layer.

llvm/examples/Kaleidoscope/BuildingAJIT/Chapter3/KaleidoscopeJIT.h

@@ -120,7 +120,7 @@ public:
     if (!RT)
       RT = MainJD.getDefaultResourceTracker();
 
-    return OptimizeLayer.add(RT, std::move(TSM));
+    return CODLayer.add(RT, std::move(TSM));
   }
 
   Expected<JITEvaluatedSymbol> lookup(StringRef Name) {