[debuginfo] Include dwo_name in the split unit to improve dwp diagnostics

When multiple DWP files are merged together and duplicate DWO IDs are
found it's currently difficult to give an actionable error message - the
DW_AT_name of the CU could be provided, but might be identical (if the
same source file is built into two different configurations), which
doesn't help the user identify the problem.

When no intermediate DWP files are generated, the path to the two DWO
files could be provided - but is lost once the DWOs are merged into a
DWP.

So, include the name of the DWO (dwo_name) in the split file so that
collissions involving a source CU from a DWP can be better diagnosed.

(improvements to llvm-dwp using this to come shortly)

llvm-svn: 264316