make descriptors typesafe

```py
class A:
pass
class D(A):
def __get__(a, b, c): return None # no error
class C:
d: A = D()
reveal_type(A.d) # A, at runtime it will be None
```

But you should still be able to return something different if you aren't subtyping another class:
```py
class A:
def __get__(a, b, c): return None

class C:
a = A()
reveal_type(C.a) # None
```

This would mean that `FunctionType` would not extend `Callable`,
- see #614

Start by investigating the type checker's descriptor handling and read the related discussion in #614. Use the two examples in this issue to define the expected reveal_type results for subclassed and standalone descriptors; the work is done when those cases are typed as shown and FunctionType no longer extends Callable as described.

Written by the indexing model from the issue text.