[ThinLTO] Migrate ThinLTOBitcodeWriter to the new PM.

Summary: Also see D33429 for other ThinLTO + New PM related changes.

Reviewers: davide, chandlerc, tejohnson

Subscribers: mehdi_amini, Prazek, cfe-commits, inglorion, llvm-commits, eraman

Differential Revision: https://reviews.llvm.org/D33525

llvm-svn: 304378