[ThinLTO] Wire up ThinLTO and new PM

Summary: This patch teaches clang to use and propagate new PM in ThinLTO.

Reviewers: davide, chandlerc, tejohnson

Subscribers: mehdi_amini, Prazek, inglorion, cfe-commits

Differential Revision: https://reviews.llvm.org/D33692

llvm-svn: 304496