[lldb] Delete the SharingPtr class

Summary:
The only use of this class was to implement the SharedCluster of ValueObjects.
However, the same functionality can be implemented using a regular
std::shared_ptr, and its little-known "sub-object pointer" feature, where the
pointer can point to one thing, but actually delete something else when it goes
out of scope.

This patch reimplements SharedCluster using this feature --
SharedClusterPointer::GetObject now returns a std::shared_pointer which points
to the ValueObject, but actually owns the whole cluster. The only change I
needed to make here is that now the SharedCluster object needs to be created
before the root ValueObject. This means that all private ValueObject
constructors get a ClusterManager argument, and their static Create functions do
the create-a-manager-and-pass-it-to-value-object dance.

Reviewers: teemperor, JDevlieghere, jingham

Subscribers: mgorny, jfb, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74153