[mlir] Use std::optional instead of llvm::Optional (NFC)

This patch replaces (llvm::|)Optional< with std::optional<.  I'll post
a separate patch to remove #include "llvm/ADT/Optional.h".

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorCodegen.cpp

@@ -134,8 +134,9 @@ Value sizeAtStoredDim(OpBuilder &builder, Location loc,
 
 static void createPushback(OpBuilder &builder, Location loc,
                            MutSparseTensorDescriptor desc,
-                           SparseTensorFieldKind kind, Optional<unsigned> dim,
-                           Value value, Value repeat = Value()) {
+                           SparseTensorFieldKind kind,
+                           std::optional<unsigned> dim, Value value,
+                           Value repeat = Value()) {
   Type etp = desc.getMemRefElementType(kind, dim);
   Value field = desc.getMemRefField(kind, dim);
   StorageSpecifierKind specFieldKind = toSpecifierKind(kind);