llvm-dwp: Include the dwo name (if available) when diagnosing duplicate CU IDs from dwp input files

If you're building dwps from other dwps, it can be hard to track down a
duplicate CU ID if it comes from two compilations of the same file in
different modes, etc. By including the .dwo path (which is hopefully
more unique than the file path) it can help track down where the
duplicates came from.

llvm-svn: 264520