Restore with fix "[ThinLTO] Ensure we always select the same function copy to import"

This restores r308078/r308079 with a fix for bot non-determinisim (make
sure we run llvm-lto in single threaded mode so the debug output doesn't get
interleaved).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308114 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed