[lldb] Add deref support to libc++ unique_ptr synthetic

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

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

Without this change, the command would have to be written as `v *thing_up.__value_` or v thing_up.__value_->m_id` which exposes internal structure and is more clumsy to type.

Additionally, the existing tests were updated. See also https://reviews.llvm.org/D97165 which added deref support for `std::shared_ptr`.

Differential Revision: https://reviews.llvm.org/D97524