[Attributor] CGSCC pass should not recompute results outside the SCC (reapply)

When we run the CGSCC pass we should only invest time on the SCC. We can
initialize AAs with information from the module slice but we should not
update those AAs. We make an exception for are call site of the SCC as
they are helpful providing information for the SCC.

Minor modifications to pointer privatization allow us to perform it even
in the CGSCC pass, similar to ArgumentPromotion.