[ThinLTO] Implement -pass-remarks-output in ThinLTOCodeGenerator

Summary:
This will also be added to the LTO API, right now this will
bring ThinLTO on par with Monolithic LTO on Darwin.

Reviewers: anemet

Subscribers: tejohnson, llvm-commits

Differential Revision: https://reviews.llvm.org/D26886

llvm-svn: 287450