[mlir-tblgen] Fix non-deterministic generating static verifier in DRR.

Use SetVector instead of DenseSet to ensure we always generate the same
name for the same function. This issue is found in
https://github.com/llvm/llvm-project/issues/53768.

Reviewed By: quinnp, rdzhabarov

Differential Revision: https://reviews.llvm.org/D120514