[dsymutil] Fix handling of empty CUs in LTO links.

r288399 introduced the DIEUnit class, and in the process broke
the corner case where dsymutil generates an empty CU during an
LTO link. This restores the logic and adds a test for the corner
case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294618 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed