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.
answered Mar 14, 2016 at 03:14 PM
Kev Riley ♦♦