[v3] Extract Concepts from v2 exercise: luhn

Extract the list of concepts required to solve the v2 C++ exercise: `luhn`. For each concept, describe why it is required to solve the exercise. Keep in mind that an exercise can have multiple approaches (you can find community solutions here: `https://exercism.io/tracks/cpp/exercises/luhn/solutions`). The goal of this task is to help determine the total list of concepts that make up `luhn`.

The list of concepts should be stored in the following document: `languages/cpp/reference/exercise-concepts/luhn.md`. [Use this link to create a PR to add this document][pr-link].

[pr-link]: ../new/master?filename=languages/cpp/reference/exercise-concepts/luhn.md

Read the v2 C++ luhn exercise and review the community solutions at the linked Exercism page to identify all approaches and required concepts. Record the concepts and why each is needed in languages/cpp/reference/exercise-concepts/luhn.md; done means the document covers the full list without omitting concepts used by alternative solutions.

Written by the indexing model from the issue text.