[Attributor] Introduce a state wrapper class

Summary:
The wrapper reduces boilerplate code and also provide a nice way to
determine the state type used by an abstract attributes statically via
AAType::StateType.

This was already discussed as part of the review of D65711.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65786

llvm-svn: 368224