[lldb] Add deref support and tests to shared_ptr synthetic

Add `frame variable` dereference suppport to libc++ `std::shared_ptr`.

This change allows for commands like `v *thing_sp` and `v thing_sp->m_id`. These
commands now work the same way they do with raw pointers. This is done by adding an
unaccounted for child member named `$$dereference$$`.

Also, add API tests for `std::shared_ptr`, previously there were none.

Differential Revision: https://reviews.llvm.org/D97165