AGLA always has good advice. So I agree with what he said.
But here's food for thought. Around here, nurseries keep plants in pots for years and years outside. Not even in greenhouses. Only the more sensitive plants are kept in greenhouses. Most of them, including lilacs, are kept in pots outdoors all winter long. And they come back just fine the next spring and then they sell the plants to guys like me.
It's probably safe to heel it in. But I think it would likely survive in the pot just fine too.