fix: Allow ClassVar descriptors to be assigned to from instances

[bzoracler]

Fixes #14969. Allows the following:

from typing import Any, ClassVar

class D:
    def __set__(self, inst: Any, value: int) -> None: pass

class A:
    x: ClassVar[D] = D()

a = A()
a.x = 0  # OK
a.x = '0'  # E: Incompatible types in assignment (expression has type "str", variable has type "int")

Currently, a.x = 0 errors with Cannot assign to class variable "x" via instance [misc] (see mypy Playground).

• The example in Descriptors and ClassVars #14969 will still error because ClassVar there is not specified with a type (an unannotated ClassVar is equivalent to ClassVar[Any]).
• Unlike in other annotation contexts, I didn't think it was a good idea to be permissive with Any, due to the potential creep of false negatives.

_{Originally created as part of PR #17170, but I forgot about that PR and deleted the fork.}

[bzoracler]

This error is in imitation of the following:

var: str | list[object] | int | None
var.append  # E: Item "None" of "str | list[object] | int | None" has no attribute "append" [union-attr] \
            # E: Item "int" of "str | list[object] | int | None" has no attribute "append" [union-attr] \
            # E: Item "str" of "str | list[object] | int | None" has no attribute "append" [union-attr]

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅