Reapply "[Cloning] Take another pass at properly cloning debug info"

This was rL304226, reverted in 304228 due to a clang assertion failure
on the build bots. That problem should have been addressed by clang
commit rL304470.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304488 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed