[mlir][sparse] replace support lib conversion with actual MLIR codegen

Rationale:
Passing in a pointer to the memref data in order to implement the
dense to sparse conversion was a bit too low-level. This revision
improves upon that approach with a cleaner solution of generating
a loop nest in MLIR code itself that prepares the COO object before
passing it to our "swiss army knife" setup.  This is much more
intuitive *and* now also allows for dynamic shapes.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D108491