[mlir][sparse] replace ad-hoc MemRef struct with CRunnerUtils definition

This revision removes the ad-hoc MemRefs that were needed using the old
ABI (when we still passed by value) and replaces them with the shared
StridedMemRef definitions of CRunnerUtils (possible now that we pass by
pointer). This avoids code duplication and makes sure we have a consistent
view of strided memory references in all our support libraries.

Reviewed By: jsetoain

Differential Revision: https://reviews.llvm.org/D110221