[mlir][openacc] Add host_data operation

The acc.host_data operation models the OpenACC
host_data construct (2.8). The host_data construct
defines a region where the address of data in device memory
available on the host. The operation is modeled in a similar way
than acc.data operation.

Reviewed By: razvanlupusoru, jeanPerier

Differential Revision: https://reviews.llvm.org/D150289