[mlir] Use std::nullopt instead of None in comments (NFC)

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/Bytecode/Reader/BytecodeReader.cpp

@@ -451,7 +451,8 @@ struct BytecodeOperationName {
   BytecodeOperationName(BytecodeDialect *dialect, StringRef name)
       : dialect(dialect), name(name) {}
 
-  /// The loaded operation name, or None if it hasn't been processed yet.
+  /// The loaded operation name, or std::nullopt if it hasn't been processed
+  /// yet.
   Optional<OperationName> opName;
 
   /// The dialect that owns this operation name.