Now we need to provide a location when call `load()`, e.g.
```python
with Context(), Location.unknown():
MyDialect.load()
```
But it's actually weird: IRDL is just an implementation details, so for
users they don't know why they need to provide a location for loading a
dialect, which is unrelated to constructing an IR module.
This PR made location optional for dialect loading.
eb687fb106