[LLDB][DataFormatter] Add data formatter for libcxx std::unordered_map iterator

This patch adds a formatter for libcxx's `std::unordered_map` iterators.
The implementation follows a similar appraoch to the `std::map` iterator
formatter. I was hesistant about coupling the two into a common
implementation since the libcxx layouts might change for one of the
the containers but not the other.

All `std::unordered_map` iterators are covered with this patch:
1. const/non-const key/value iterators
2. const/non-const bucket iterators

Note that, we currently don't have a formatter for `std::unordered_map`.
This patch doesn't change that, we merely add support for its iterators,
because that's what Xcode users requested. One can still see contents
of `std::unordered_map`, whereas with iterators it's less ergonomic.

**Testing**

* Added API test

Differential Revision: https://reviews.llvm.org/D129364

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