[Attributor] Introduce AA[Intra/Inter]Reachability

We had two AAs for reachability but it was very cumbersome to extend
them. We also had some fallback to use LLVM-core mechanisms and cache
the result. The new design shares the query code and interface nicely
between AAIntraFnReachability and AAInterFnReachability.

As part of the rewrite we also added the ExclusionSet to the queries.