[mlir][openacc] Use new private representation in acc.serial

Update acc.serial private operands list to use the new design
introduced in D150622.

Test in flang/test/Lower/OpenACC/acc-parallel.f90 and
flang/test/Lower/OpenACC/acc-parallel-loop.f90 are temporarly
disabled and will be enabled with updated lowering in the next
patch.

Depends on D150971

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D150972