[mlir][sparse] Making deletion of SparseTensorType copy-assignment explicit

[wrengr]

Summary

The copy assignment is already implicitly deleted, but making it explicit helps clean up compilation error messages.

mlir/include/mlir/Dialect/SparseTensor/IR/SparseTensorType.h

SparseTensorType(RankedTensorType rtp)
         dim2lvl(enc.hasIdDimOrdering() ? AffineMap() : enc.getDimOrdering()) {
     assert((!isIdentity() || getDimRank() == lvlRank) && "Rank mismatch");
   }
 
   SparseTensorType(ShapedType stp, SparseTensorEncodingAttr enc)
       : SparseTensorType(
             RankedTensorType::get(stp.getShape(), stp.getElementType(), enc)) {}
 
+  // Copy-assignment would be implicitly deleted (because our fields
+  // are const), so we explicitly delete it for clarity.
+  SparseTensorType &operator=(const SparseTensorType &) = delete;
+
   /// Constructs a new `SparseTensorType` with the same dimension-shape
   /// and element type, but with the encoding replaced by the given encoding.
   SparseTensorType withEncoding(SparseTensorEncodingAttr newEnc) const {
     return SparseTensorType(rtp, newEnc);
   }
 
   /// Constructs a new `SparseTensorType` with the same dimension-shape
   /// and element type, but with the encoding replaced by