llvm-dwp: provide diagnostics for duplicate DWO IDs

These diagnostics aren't perfect - in the case of merging several dwos
into dwps and those dwps into more dwps - just getting the message about
the original source file name might not be much help (since it's the
same in both dwos, by definition - but doesn't tell you which chain of
dwps to backtrack)

It might be worth adding the DW_AT_dwo_id to the split debug info to
improve the diagnostic experience - might help track down the duplicates
better.

llvm-svn: 261988