PR16091: Error when attempting to emit debug info for undeduced auto return types

Perhaps we should just suppress this, rather than erroring, but since we
have the infrastructure for it I figured I'd use it - if this is
determined to be not the right thing we should probably remove that
infrastructure entirely. I guess it's lying around from the early days
of implementing debug info support.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182673 91177308-0d34-0410-b5e6-96231b3b80d8