[mlir][sparse][gpu] first implementation of the GPU libgen approach

[mlir][sparse][gpu] first implementation of the GPU libgen approach

The sparse compiler now has two prototype strategies for GPU acceleration:

* CUDA codegen: this converts sparsified code to CUDA threads
* CUDA libgen: this converts pre-sparsified code to cuSPARSE library calls

This revision introduces the first steps required for the second approach.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D150170