[MLIR] Fix warnings in AttrTypeSubElements.h

Fix warnings in AttrTypeSubElements.h (below) with GCC 9.4.0.

```
mlir/lib/IR/ExtensibleDialect.cpp:443:62:   required from here
mlir/include/mlir/IR/AttrTypeSubElements.h:412:37: warning: parameter ‘derived’ set but not used [-Wunused-but-set-parameter]
  412 | void walkImmediateSubElementsImpl(T derived,
      |                                   ~~^~~
```

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D146107