Fix member access on generic classes

[ilevkivskyi]

Fixes #3645
Fixes #1337
Fixes #5664

The fix is straightforward, I just add/propagate the bound type variable values by mapping to supertype.

I didn't find any corner cases with class methods, and essentially follow the same logic as when we generate the callable from __init__ for generic classes in calls like C() or C[int]().

For class attributes there are two things I fixed. First we used to prohibit ambiguous access:

class C(Generic[T]):
    x: T
C.x  # Error!
C[int].x  # Error!

but the type variables were leaking after an error, now they are erased to Any. Second, I now make an exception and allow accessing attributes on Type[C], this is very similar to how we allow instantiation of Type[C] even if it is abstract (because we expect concrete subclasses there), plus this allows accessing variables on cls (first argument in class methods), for example:

class C(Generic[T]):
    x: T
    def get(cls) -> T:
        return cls.x  # OK

(I also added a bunch of more detailed comments in this part of code.)

[msullivan]

This seems good to me. One additional comment request

[msullivan]

Could you add some comments about what is going on here?

[ilevkivskyi]

I added a detailed comment (and even found a little bug while writing it).

[ilevkivskyi]

I double-checked that everything is OK with internal code bases.