[Attributor] Introduce the IRAttribute helper struct

Summary:
Certain properties, e.g., an AttrKind, are not shared among all abstract
attributes. This patch extracts the functionality into a helper struct.

Reviewers: uenoku, sstefan1

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65712

llvm-svn: 367953