This PR adds basic support for defining regions in Python-defined
dialects. Example usage:
```python
class TestRegion(Dialect, name="ext_region"):
pass
class IfOp(TestRegion.Operation, name="if"):
cond: Operand[IntegerType[1]]
then: Region
else_: Region
```
Current limitations:
* We can’t specify region constraints yet (e.g., number of blocks or
block argument types). This will be addressed as a follow-up task.
* We can’t mark an op as a `Terminator` or `NoTerminator` yet. This
depends on `DynamicOpTraits` (#177735) and Python-side trait API
support, and will be implemented in a follow-up PR.
This is the first PR after splitting off #179032.
This is a follow-up PR of #169045.
---------
Co-authored-by: Rolf Morel <rolfmorel@gmail.com>
cb274ea176