[Devirtualization] insert placement new barrier with -O0

Summary:
To not break LTO with different optimizations levels, we should insert
the barrier regardles of optimization level.

Reviewers: rjmccall, rsmith, mehdi_amini

Reviewed By: mehdi_amini

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D32401

llvm-svn: 303488