Fixes python#18505
Fixes python#16757

Fixing the crash is trivial, we simply give an error on something like
`type X = X`, instead of crashing. But then I looked at what people
actually want to do, they want to create an alias to something currently
in scope (not a meaningless no-op alias). Right now we special-case
classes/aliases to allow forward references (including recursive type
aliases). However, I don't think we have any clear "scoping rules" for
forward references. For example:
```python
class C:
    Y = X
    class X: ...
class X: ...
```
where `Y` should point to, `__main__.X` or `__main__.C.X`? Moreover,
before this PR forward references can take precedence over real
references:
```python
class X: ...
class C:
    Y = X  # this resolves to __main__.C.X
    class X: ...
```
After some thinking I found this is not something I can fix in a simple
PR. So instead I do just two things here:
* Fix the actual crashes (and other potential similar crashes).
* Add minimal change to accommodate the typical use case.