[Attributor] Introduce the closed world flag

The Attributor user can now set the closed world flag
(`AttributorConfig.IsClosedWorldModule` or
`-attributor-assume-closed-world`) in order to specialize call edges
based only on available callees. That means, we assume all functions are
known and hence all potential callees must be declared/defined in the
module. We will use this for GPUs and LTO cases, but for now the user
has to set it via a flag.