[mlir][OpenACCToLLVM] Add pass option to emit opaque pointers

Part of https://discourse.llvm.org/t/rfc-switching-the-llvm-dialect-and-dialect-lowerings-to-opaque-pointers/68179

This patch simply adds the pass option use-opaque-pointers to instruct the pass to use opaque-pointers instead of typed pointers during conversion.
The pass itself does not actually make use of any pointers or anything, so did not require any changes except simply setting the option in the type converter. The tests have also been converted to using opaque-pointers

Differential Revision: https://reviews.llvm.org/D144462