[MLIR][IRDL][Python] Fix error while composing `irdl.any_of` and `irdl.base` (#187914)

Previously, while users compose `irdl.any_of` and `irdl.base`, e.g.
```mlir
module {
  irdl.dialect @ext_attr_in_op {
    irdl.operation @op_with_attr {
      %0 = irdl.base "#builtin.integer"
      %1 = irdl.base "#builtin.string"
      %2 = irdl.any_of(%0, %1)
      irdl.attributes {"a" = %2}
    }
  }
}
```

The program will crash due to `llvm_unreachable("unknown IRDL
constraint")`.

This PR implements `getBases(..)` for `irdl::BaseOp` to make this work.
This make fields like `attr: IntegerAttr | StringAttr` work in
Python-defined dialects, which led to a crash previously.

GitOrigin-RevId: 6dabcef0b3ffa0beadd426e0fd56c61c45b5b396