[CODEGEN] Fix Metal codegen when storage scope is needed (#469)

src/codegen/codegen_c.cc

@@ -142,6 +142,10 @@ std::string CodeGenC::GetBufferRef(
     os << "*)(";
     if (!HandleTypeMatch(buffer, t.element_of())) {
       os << '(';
+      if (scope.length() != 0) {
+        PrintStorageScope(scope, os);
+      }
+      os << ' ';
       PrintType(t.element_of(), os);
       os << "*)";
     }

src/codegen/codegen_metal.cc

@@ -192,6 +192,8 @@ void CodeGenMetal::PrintStorageScope(
     os << "device";
   } else if (scope == "shared") {
     os << "threadgroup";
+  } else {
+    os << "thread";
   }
 }