By extending this class, a class can define its own `getLocation`
predicate without participating in the dispatch hierarchy of
`getLocation` as defined on `Element`. Classes wanting to override their
location previously had to define `getURL` or `hasLocationInfo` instead
and rely on these predicates not being defined on future versions of
`Element`.
This avoids using a raw db type.
It is possible for `SuppressionComment` and `SuppressionScope` to have
different locations because `SuppressionScope` defines `hasLocationInfo`
as a new rootdef whereas `SuppressionComment` only responds to
`getLocation` that it inherited. In interpretation of query results, a
`hasLocationInfo` predicate is preferred over `getLocation` if it
exists.
