analyzer issue

Playing with this today, the issues discussed in PR7406 seem to stem from
two causes:

1. AttrNonNullChecker doesn't expect the value of a nonnull argument to be
unknown when it's checked. Patch here:

2. The additive folding code in SimpleSValuator and the constraint manager
doesn't handle the case where the constants are of different types
(unsigned, long long, etc). Patch here:

#2 in particular could use a review, since it's a nontrivial fix (a number
of type conversions). And of course it was me who didn't get this right the
first time around. Ted? Zhongxing?


Both patches look good to me.

Both of these patches look great.