Complain about inferring UninhabitedType for a variable

[JukkaL]

These don't really make sense as variable types, most of the time.
Also, don't turn them into partial None types, since UninhabitedType
and None no longer have any correspondence.

Fixes #4104.

[JukkaL]

Don't merge yet -- I want to fix an issue in internal Dropbox code first.

[ilevkivskyi]

So does this fix the problem with:

def f() -> T: pass

x = f()
reveal_type(x)  # None

If yes then maybe it makes sense to add a test for this?

[JukkaL]

Yes, there is an existing test case that was skipped (testUnderspecifiedInferenceResult).

[JukkaL]

Updated the description to mention that this fixes #4104.

[ilevkivskyi]

Ah, OK, but then #4059 should be updated after you merge this, since the reveal_type you added will start working.