Add a libLTO API to stop/restart ThinLTO between optimizations and CodeGen

This allows the linker to instruct ThinLTO to perform only the
optimization part or only the codegen part of the process.

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265113 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed