[mlir][sparse] Using non-empty function name suffix for OverheadType::kIndex

The trick of using an empty token in the `FOREVERY_O` x-macro relies on preprocessor behavior which is only standard since C99 6.10.3/4 and C++11 N3290 16.3/4 (whereas it was undefined behavior up through C++03 16.3/10).  Since the `ExecutionEngine/SparseTensorUtils.cpp` file is required to be compile-able under C++98 compatibility mode (unlike the C++11 used elsewhere in MLIR), we shouldn't rely on that behavior.

Also, using a non-empty suffix helps improve uniformity of the API, since all other primary/overhead suffixes are also non-empty.  I'm using the suffix `0` since that's the value used by the `SparseTensorEncoding` attribute for indicating the index overhead-type.

Depends On D126720

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D126724