[analyzer] Also transform "a < b" to "(b - a) > 0" in the constraint manager.

We can support the full range of comparison operations between two locations
by canonicalizing them as subtraction, as in the previous commit.

This won't work (well) if either location includes an offset, or (again)
if the comparisons are not consistent about which region comes first.

<rdar://problem/13239003>

llvm-svn: 177803