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
d1e9d0b27f