[Attributor] Look through indirect calls (#65197)

Through the new `Attributor::checkForAllCallees` we can look through
indirect calls and visit all potential callees if they are known. Most
AAs will do that implicitly now via `AACalleeToCallSite`, thus, most AAs
are able to deal with missing callees for call site IR positions.

Differential Revision: https://reviews.llvm.org/D112290