[Attributor] AAFunctionReachability, Handle CallBase Reachability.

This patch makes it possible to query callbase reachability
(Can a callbase reach a function Fn transitively).
The patch moves the reachability query handling logic to a member class,
this class will have more users within the AA once we add other function
reachability queries.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106402