[Attributor] Use abstract call sites for call site callback

Summary:
When we iterate over uses of functions and expect them to be call sites,
we now use abstract call sites to allow callback calls.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, hfinkel, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67871

llvm-svn: 373985