[mlir][sparse] Moving lexOrder from SparseTensorCOO to Element

[mlir][sparse] Moving lexOrder from SparseTensorCOO to Element

This is the more logical place for the function to live.  If/when we factor out a separate class for just the `Coordinates` themselves, then the definition should be moved to `Coordinates::lexOrder` (and `Element::lexOrder` would become a thin wrapper delegating to that function).

This is (tangentially) work towards fixing: https://github.com/llvm/llvm-project/issues/51652

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D122057