[mlir][openacc] Model acc cache directive as data entry operands on acc.loop (#65521)

The `cache` directive may appear at the top of (inside of) a loop. It
specifies array elements or subarrays that should be fetched into the
highest level of the cache for the body of the loop.

The `cache` directive is modeled as a data entry operands attached to
the acc.loop operation.