[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
