Add IfaceType deduplication table to interface file serialisation

Although we do not really need it in the interface file serialisation,
as the deserialisation uses `getWithUserData`, we need to mirror the
structure `getWithUserData` expects. Thus, we write essentially an empty
`IfaceType` table at the end of the file, as the interface file doesn't
reference `IfaceType`.

(cherry picked from commit c9bc29c6a708483d2abc3d8ec9262510ce87ca61)