Pmc: Do inhabitation test for unlifted vars (#20631)

Although I thought we were already set to handle unlifted datatypes correctly, it appears we weren't. #20631 (closed) showed that it's wrong to assume vi_bot=IsNotBot for VarInfos of unlifted types from their inception if we don't follow up with an inhabitation test to see if there are any habitable constructors left. We can't trigger the test from emptyVarInfo, so now we instead fail early in addBotCt for variables of unlifted types.

Fixed #20631 (closed).