Switch to `String(reflecting:)` in descriptors

[dimitribouniol]

String(describing:) unfortunately will change a complex type like AccountStore.Item.ID into just ID — it might be better to use String(reflecting:) instead, which should provide the fully qualified name. We need to make sure this works well with local types and migrations though, since we may hit those quite often.

See: https://forums.swift.org/t/getting-the-more-fully-qualified-name-of-a-type/14375/10