[mlir] Silence a few -Wunused-but-set-parameter warnings

Some older versions of gcc hit false positives here:

/workdir/llvm-project/mlir/include/mlir/IR/Attributes.h:391:49: warning: parameter 'ty' set but not used [-Wunused-but-set-parameter]
  391 |   static inline bool isPossible(mlir::Attribute ty) {

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81676