[mlir][SCFToOpenMP] Add pass option to emit LLVM opaque pointers

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

There were luckily only very few changes that had to be made. To allow users to also specify the pass option from C++ code I have also migrated the pass to use autogenerated constructors to autogenerate a pass option struct.

Differential Revision: https://reviews.llvm.org/D143855