<rdar://problem/15776874>

ValueObjectPrinter could enter an infinite loop while trying to display an aptly formed ValueObject: a reference, with a child of some pointer type, such that the pointees chain ended up pointing back to some part of itself - a pointer to itself being the simplest such case

Fixed here by only setting a pointer depth when needed, and ensuring that we won't overflow and wrap the pointer depth when it's zero.

llvm-svn: 200247