[analyzer] Allow tracknullOrUndef look through the ternary operator even when condition is unknown

Improvement of r178684 and r178685.

Jordan has pointed out that I should not rely on the value of the condition to know which expression branch
has been taken. It will not work in cases the branch condition is an unknown value (ex: we do not track the constraints for floats).
The better way of doing this would be to find out if the current node is the right or left successor of the node
that has the ternary operator as a terminator (which is how this is done in other places, like ConditionBRVisitor).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178701 91177308-0d34-0410-b5e6-96231b3b80d8