[LLDB] Do not dereference promise pointer in `coroutine_handle` pretty printer

So far, the pretty printer for `std::coroutine_handle` internally
dereferenced the contained frame pointer displayed the `promise`
as a sub-value. As noticed in https://reviews.llvm.org/D132624
by @labath, this can lead to an endless loop in lldb during printing
if the coroutine frame pointers form a cycle.

This commit breaks the cycle by exposing the `promise` as a pointer
type instead of a value type. The depth to which the `frame variable`
and the `expression` commands dereference those pointers can be
controlled using the `--ptr-depth` argument.

Differential Revision: https://reviews.llvm.org/D132815