Neither DBCC FREEPROCCACHE nor DBCC FREESYSTEMCACHE('SQL Plans') do anything to free CACHESTORE_SQLCP memory
I upgraded to 13.00.4422.0.v1 (SQL Server 2016 SP1) and restarted.

So far, I am able to clear SQL Plan memory using DBCC FREESYSTEMCACHE('SQL Plans'); command!

Time will tell to see if it uses the memory a little wiser now, but at least now I have a way to reset it if its gets too bloated again.