[flang][OpenMP] Add support for copyprivate (#80485)

Add initial handling of OpenMP copyprivate clause in Flang.

When lowering copyprivate, Flang generates the copy function
needed by each variable and builds the appropriate
omp.single's CopyPrivateVarList.

This is patch 3 of 4, to add support for COPYPRIVATE in Flang.
Original PR: https://github.com/llvm/llvm-project/pull/73128