refactor COFF linker to use new LTO API

Summary: The COFF linker previously implemented link-time optimization using an API which has now been marked as legacy. This change refactors the COFF linker to use the new LTO API, which is also used by the ELF linker.

Reviewers: pcc, ruiu

Reviewed By: pcc

Subscribers: mgorny, mehdi_amini

Differential Revision: https://reviews.llvm.org/D29059

llvm-svn: 293967