[llvm-reduce] add ReduceAttribute delta pass

Summary:
The output from llvm-reduce still has significantly more attributes than
bugpoint does.  Teach llvm-reduce to remove attributes.

Reviewers: diegotf, dblaikie, george.burgess.iv

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73853