llvm-dwp: Streamline duplicate DWO ID diagnostic handling

Actually use the error return path rather than printing the duplicate
information then a separate error. But also just tidy up/deduplicate
some of the code for generating the diagnostic text.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269692 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed