[BOLT][DWARF][NFC] Set initial offset of DIE

[ayermolo]

Summary

Setting initial offset of DIE to input DIE. This is to make "printf" debugging
easier.

bolt/lib/Core/DIEBuilder.cpp

if (DWARFUnitInfo.IsConstructed && DieInfo.Die)
       return DieInfo.Die;
   } else {
     Idx = allocDIE(U, DDie, getState().DIEAlloc, UnitId);
   }
 
   DIEInfo &DieInfo = getDIEInfo(UnitId, *Idx);
 
   uint64_t Offset = DDie.getOffset();
+  // Just for making debugging easier.
+  // DIE dump is not very useful.
+  // It's nice to know original offset from which this DIE was constructed.
+  DieInfo.Die->setOffset(Offset);
   uint64_t NextOffset = Offset;
   DWARFDataExtractor Data = U.getDebugInfoExtractor();
   DWARFDebugInfoEntry DDIEntry;
 
   if (DDIEntry.extractFast(U, &NextOffset, Data, U.getNextUnitOffset(), 0))
     assert(NextOffset - U.getOffset() <= Data.getData().size() &&
            "NextOffset OOB");