[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

mlir/include/mlir/IR/Attributes.h

@@ -392,6 +392,7 @@ struct CastInfo<To, From,
     /// Return a constant true instead of a dynamic true when casting to self or
     /// up the hierarchy.
     if constexpr (std::is_base_of_v<To, From>) {
+      (void)ty;
       return true;
     } else {
       return To::classof(ty);

mlir/include/mlir/IR/Types.h

@@ -387,6 +387,7 @@ struct CastInfo<
     /// Return a constant true instead of a dynamic true when casting to self or
     /// up the hierarchy.
     if constexpr (std::is_base_of_v<To, From>) {
+      (void)ty;
       return true;
     } else {
       return To::classof(ty);

mlir/include/mlir/IR/Value.h

@@ -576,6 +576,7 @@ struct CastInfo<
     /// Return a constant true instead of a dynamic true when casting to self or
     /// up the hierarchy.
     if constexpr (std::is_base_of_v<To, From>) {
+      (void)ty;
       return true;
     } else {
       return To::classof(ty);