'IntEnum' values from different types compare equal

For instance:

```python
from google.cloud.firestore_v1.types import StructuredQuery

greater_than = StructuredQuery.FieldFilter.Operator.GREATER_THAN
is_null = StructuredQuery.UnaryFilter.Operator.IS_NULL

assert greater_than != is_null  # fails
```

This "false equivalence" is the source of https://github.com/googleapis/python-firestore/issues/429