[v3] Extract Concepts from v2 exercise: allergies

Extract the list of concepts required to solve the v2 C++ exercise: `allergies`. 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/allergies/solutions`). The goal of this task is to help determine the total list of concepts that make up `allergies`.

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

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

Start by reviewing the v2 C++ allergies exercise and the linked community solutions to identify the approaches used. Create languages/cpp/reference/exercise-concepts/allergies.md, listing every required concept and explaining why it is needed; the document is complete when it covers the concepts across the possible approaches.

Rédigé par le modèle d'indexation à partir du texte de l'issue.