[mlir] Fix warning `missing base in copy ctor`

This suppresse annoying warning when building mlir.
```
 warning: base class ‘class mlir::PassWrapper<{anonymous}::TestStatisticPass,
  mlir::OperationPass<void> >’ should be explicitly initialized in the copy constructor [-Wextra]
```

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D126209