[lldb][DataFormatter] Simplify libc++ std::map::iterator formatter (#97713)

Depends on https://github.com/llvm/llvm-project/pull/97687

Similar to https://github.com/llvm/llvm-project/pull/97579, this patch
simplifies the way in which we retrieve the key/value pair of a
`std::map` (in this case of the `std::map::iterator`).

We do this for the same reason: not only was the old logic hard to
follow, and encoded the libc++ layout in non-obvious ways, it was also
fragile to alignment miscalculations
(https://github.com/llvm/llvm-project/pull/97443); this would break once
the new layout of std::map landed as part of
https://github.com/llvm/llvm-project/issues/93069.

Instead, this patch simply casts the `__iter_pointer` to the
`__node_pointer` and uses a straightforward
`GetChildMemberWithName("__value_")` to get to the key/value we care
about.

We can eventually re-use the core-part of the `std::map` and
`std::map::iterator` formatters. But it will be an easier to change to
review once both simplifications landed.

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