[lldb][DataFormatter] unordered_map: account for new libc++ __hash_node layout (#68574)

Since D101206 (`ba79fb2e`) the `__hash_node::__value_`
member is wrapped in an anonymous union. `ValueObject::GetChildMemberWithName` doesn't see
through the union.

This patch accounts for this possible new layout by getting a handle to
the union before doing the by-name `__value_` lookup.