[nfc] [lldb] Add Dwo assert

[jankratochvil]

Summary

I had it in my local patchset and I did not want to drop it, OK for check-in?

[labath]

Hm... I'm not sure about that... I mean, there's not much use for it, and we probably aren't doing anything like that right now, but I don't see anything inherently wrong about querying a skeleton unit for a DIE (even though it should only contain the root die).

[jankratochvil]

OK, dropped.

lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp

 // Get the DIE (Debug Information Entry) with the specified offset by first
 // checking if the DIE is contained within this compile unit and grabbing the
 // DIE from this compile unit. Otherwise we grab the DIE from the DWARF file.
 DWARFDIE
 DWARFUnit::GetDIE(dw_offset_t die_offset) {
   if (die_offset == DW_INVALID_OFFSET)
     return DWARFDIE(); // Not found
 
+  // SymbolFileDWARF::GetDIE should have found the dwo file already.
+  lldbassert(!GetDwoSymbolFile());
+
   if (!ContainsDIEOffset(die_offset)) {
     GetSymbolFileDWARF().GetObjectFile()->GetModule()->ReportError(
         "GetDIE for DIE 0x%" PRIx32 " is outside of its CU 0x%" PRIx32,
         die_offset, GetOffset());
     return DWARFDIE(); // Not found
   }
 
   ExtractDIEsIfNeeded();