[Kaleidoscope][BuildingAJIT] Fix a bug in the symbol resolver in Chapter2.

Symbol resolution should be done on the top layer of the stack unless there's a
good reason to do otherwise. In this case it would have worked because
OptimizeLayer::addModuleSet eagerly passes all modules down to the
CompileLayer, meaning that searches in CompileLayer will find the definitions.
In later chapters where the top layer's addModuleSet isn't a pass-through, this
would break.

llvm-svn: 270899

llvm/examples/Kaleidoscope/BuildingAJIT/Chapter2/KaleidoscopeJIT.h

@@ -71,7 +71,7 @@ public:
     // Lambda 2: Search for external symbols in the host process.
     auto Resolver = createLambdaResolver(
         [&](const std::string &Name) {
-          if (auto Sym = CompileLayer.findSymbol(Name, false))
+          if (auto Sym = OptimizeLayer.findSymbol(Name, false))
             return RuntimeDyld::SymbolInfo(Sym.getAddress(), Sym.getFlags());
           return RuntimeDyld::SymbolInfo(nullptr);
         },