Use a SmallPtrSet rather than a SmallVector in ClusterManager.

The m_objects store in this class is only used to check whether
this ClusterManager already owns this pointer.  With a SmallVector
the is_contained call was non-linear in number of children, and for
instance printing all the elements of a 16M element std::vector
didn't complete in the time I was willing to wait for it (hours).

Since we are only doing insert & contains, some kind of set is a
better data structure.  In this patch I used SmallPtrSet.  With
that, the same array prints out in 30 seconds.  I also tried a
std::unordered_set but that was slightly slower and used a fair bit
more memory.

Other than performance, this is NFC.

Differential Revision: https://reviews.llvm.org/D131996