OpenCL private_copies attribute does not seem to work in 20.1

@AnilErinch_A_Intel Providing better explanation in the documentation is useful; however, it does not solve the underlying problem. What is required here is to give the users an extra attribute to control the on-chip memory replication factor for "supporting simultaneous work-groups". i.e. the possibility to control the number of work-groups the compiler would schedule simultaneously in a single compute unit. The functionality already exists in the compiler; it just needs to be exposed to the users in form of an attribute/pragma. I opened a support ticket with Altera/Intel about this exact problem years ago, but such attribute/pragma is yet to be provided.