Set MAttrs in LTO mode

Summary: Without this patch, MAttrs are not set.

Patch by Yin Ma

Reviewers: espindola, MaskRay, ruiu, pcc

Reviewed By: MaskRay, pcc

Subscribers: pcc, emaste, sbc100, inglorion, arichardson, aheejin, steven_wu, llvm-commits

Differential Revision: https://reviews.llvm.org/D53446

llvm-svn: 345884