Do not narrow types to Never with binder (#18972)
Fixes https://github.com/python/mypy/issues/18967
Fixes https://github.com/python/mypy/issues/16494
Fixes https://github.com/python/mypy/issues/15793
Fixes https://github.com/python/mypy/issues/12949
As you can see from updated test cases, it is kind of gray area, so
whether we go this way will depend on the `mypy_primer` results (and
also potentially on Dropbox internal code bases, where the above issue
may cause problems).
