[LLDB] Fix for synthetic children memory leak

The lifetime of a ValueObject and all its derivative ValueObjects (children, clones, etc.) is managed by a ClusterManager. These objects are only destroyed when every shared pointer to any of the managed objects in the cluster is destroyed. This means that no object in the cluster can store a shared pointer to another object in the cluster without creating a memory leak of the entire cluster. However, some of the synthetic children front-end implementations do exactly this; this patch fixes that.

Differential Revision: https://reviews.llvm.org/D68641

llvm-svn: 374195