[mlir][sparse] Adding `LvlVar` forward-declarations to `DimLvlMap::print`

This commit makes `DimLvlMap::print` consistent with `DimLvlMapParser` with respect to both the forward-decls per se, as well as the all-or-none constraint on LvlVar-bindings in LvlSpecs.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D156768