Treat type aliases as typing-only expressions (#7968)
## Summary
Given `type RecordOrThings = Record | int | str`, the right-hand side
won't be evaluated at runtime. Same goes for `Record` in `type
RecordCallback[R: Record] = Callable[[R], None]`. This PR modifies the
visitation logic to treat them as typing-only.
Closes https://github.com/astral-sh/ruff/issues/7966.
