[ThinLTOCodeGenerator] Add support for index-based WPD

This is clang part of the patch. It adds -flto-unit flag for thin LTO
builds on Mac and PS4

Differential revision: https://reviews.llvm.org/D68950


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375224 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed