[NewPassManager] Add tuning option: LoopUnrolling [clang-change]

Summary:
Use CodeGenOpts's setting for loop unrolling.
[to be coupled with D61618]

Reviewers: chandlerc

Subscribers: jlebar, dmgreen, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61620

llvm-svn: 361653