Improvements to descriptor __set__

[bzoracler]

1. Fix wrong error location when assignment errors is due to __set__. Presently, the error location appears on the right:

   # mypy: pretty=True
   
   import typing as t
   
   class desc:
       __set__ = None
       
   class A:
       prop = desc()

   >>> abc: A = A()
   >>> abc.prop = 123
                  ^~~
                  E

   e570aa1 will change the error location to be on the left:

   >>> abc: A = A()
   >>> abc.prop = 123
       ^~~~~~~~
       E

2. Fixes Descriptors and ClassVars #14969

   • The example in the issue 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.

[github-actions]

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