[mlir][openacc] Add async attribute and optional operand

OpenACC 3.2 allowed the async clause to the data construct. This patch
adds a unit attribute and an optional operand to the data operation to model
the data clause in a similar way it was added to other data operation.
The attribute models the presence of the clause without any argument. When
an argument is provided it is placed in the async operand.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D154111