[JSONExporter] Try to appease buildbot. NFC.

The compiler does not seem to able move a local variable in the
function's return statement.

llvm-svn: 338466

polly/lib/Exchange/JSONExporter.cpp

@@ -173,7 +173,7 @@ static json::Value getJSON(Scop &S) {
   }
 
   root["statements"] = std::move(Statements);
-  return root;
+  return json::Value(std::move(root));
 }
 
 static void exportScop(Scop &S) {