[lldb][DataFormatter] Move std::unordered_map::iterator formatter into LibCxxUnorderedMap.cpp (#97752)
Similar to how we moved the `std::map::iterator` formatter in
https://github.com/llvm/llvm-project/pull/97687, do the same for
`std::unordered_map::iterator`.
Again the `unordered_map` and `unordered_map::iterator` formatters try
to do very similar things: retrieve data out of the map. The iterator
formatter does this in a fragile way (similar to how `std::map` does it,
see https://github.com/llvm/llvm-project/pull/97579). Thus we will be
refactoring the `std::unordered_map::iterator` in upcoming patches.
Having it in `LibCxxUnorderedMap` will allow us to re-use some of the
logic (and we won't have to repeat some of the clarification comments).