[flang][hlfir][openacc] Generate hlfir.declare in privatization recipes

[vzakhari]

We need to follow general HLFIR lowering approach for variables in privatization recipes: https://github.com/llvm/llvm-project/pull/66099/files#diff-ae16511219ce6e2967395ff571a85894541b0e134851ced07bf150662793b667R9