[mlir][openacc][NFC] Rename ReductionOp to ReductionOperator

As we are moving on with new design for the private clause representation,
the reduction clause will aslo get remodeled. In order to new clash with
a new dedicated reduction operation, this patch rename the current
ReductionOp to ReductionOperator. It might just become obsolete once the new
design is in place.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D150711