[Attributor] Make abstract attributes stateless

To remove boilerplate, mostly passing through values to the
AbstractAttriubute base class, we extract the state into an IRPosition
helper. There is no function change intended but the IRPosition struct
will provide more functionality down the line.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65711

llvm-svn: 367952