Don't set a parent for descriptor bound methods created on __get__ access

This is a very tricky bug and requires particular circumstances to reproduce.
For `f.__get__` we have a custom binding interface that generates bound
methods on the fly whenever we encounter an `f.__get__(None, ...)` call.
On function creation, we set the name of the function as a variable in
the frame of the function's parent. e.g. for the following, we'll set
`variable` as a name in the function's locals:

   class A:
       ...
      def variable(self):  # variable will be part of A.locals
          pass

Now the bug that this commit solves requires a `__get__()` binding such
as the one mentioned as well as passing an object's dunder attribute
to `__get__`, such as `object.__eq__`. The result is that the `None`
 that was passed in `f.__get__` will have as a frame the function where
the descriptor binding method was called, which will result in `__eq__`
to be marked as a variable of the function that was called, which is
completely wrong.

As a solution we don't set any parent for descriptor bound methods,
which are created on the fly and thus the parent might be wrong altogether.

Close pylint-dev/pylint#3225

Author: PCManticore

astroid/interpreter/objectmodel.py

@@ -322,7 +322,6 @@ def infer_call_result(self, caller, context=None):
                     doc=func.doc,
                     lineno=func.lineno,
                     col_offset=func.col_offset,
-                    parent=cls,
                 )
                 # pylint: disable=no-member
                 new_func.postinit(func.args, func.body, func.decorators, func.returns)