[Attributor][NFC] Move DerefState to header and use StateWrapper

Summary: In D65402, I want to get DerefState from AADereferenceable but it was not allowed. This patch moves DerefState definition into Attributor.h and makes AADerefenceable inherit StateWrapper.

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66585

llvm-svn: 369653