type variables should be allowed on `ClassVar`s

```py
class A[T]:
t: ClassVar[T]
A[int].t = 1
```
this is obviously an error, but:
```py
class B(A[int]):
pass
B.t = 1
```
this should be allowed

Start by reproducing the two Python snippets from the issue with basedmypy and compare their diagnostics. Trace the handling of generic type variables in ClassVar annotations and inheritance; done means A[int].t is rejected while B.t is accepted.

Written by the indexing model from the issue text.