Crash with forward reference in TypedDict

This causes a crash because we take a join during the second pass of semantic analysis when MROs aren't populated yet:

```py
from mypy_extensions import TypedDict

X = TypedDict('X', {'b': 'B', 'c': int})

class B: pass
```

Originally reported by @JelleZijlstra as #3315, but separated the crash with TypedDict into this new issue.