[mlir][LinalgToLLVM] Add pass option for emitting opaque-pointers

While the pass itself doesn't directly require any changes to be compatible with opaque-pointers, it does import patterns for lowering MemRefs to LLVM.
Adding this pass option allows any users of the pass to switch to emitting opaque-pointers via the pass option

Differential Revision: https://reviews.llvm.org/D144724