[mlir][sparse] adding OverheadType::kIndex

Depends On D115008

This change opens the way for D115012, and removes some corner cases in `CodegenUtils.cpp`. The `SparseTensorAttrDefs.td` already specifies that we allow `0` bitwidth for the two overhead types and that it is interpreted to mean the architecture's native width.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D115010