llvm-dwp: Include dwo name even when the input is a dwo

This still only includes the dwo name if it's in the DW_AT_dwo_name
attribute in the split unit - though it could be improved/modified to
use the dwo name from the command line (if linking raw dwo files) or
retrieved from the DW_AT_dwo_name in the executable (when using -e).

It's useful in any case because you might have a large command line with
many files and knowing exactly which dwo files are relevant will
simplify debugging, but especially with '-e' when you didn't pass the
dwo files explicitly in nthe first place it would be quite non-obvious
where the duplicate units are coming from.