Replace an `std::unordered_map` in `async_results_merger.cpp` with an `std::array`.
The usage of `std::unordered_map` is forbidden by our conventions, and the previous code only linted successfully because of a `NOLINT` override in the code.
As the container always has just 3 members and the lookup is fast anyway, it is now turned into an `std::array`, and the NOLINT comment is removed.