I recently had the same question, so I tested it out. Procedure cache on the server that is the new primary, I saw no effect - but the proc cache on the server that was Primary and had become secondary had it's plans cleared. This was different to how the cache is managed with mirroring failover :
http://www.sqlsoldier.com/wp/sqlserver/doesamirroringfailovercleartheprocedurecache Buffer cache - as far as I am aware, failing over has no effect either way, and cache management continues to work in the normal way. It will be a warm cache as the process of syncing data modifications between AGs will have pulled data into cache, but it won't be the same on each replica.