[DebugInfo] Fix a comment and a variable name. NFC.

[ikudrin]

Summary

DebugLocListIndex keeps the index of an entry list, not the offset.

[jhenderson]

LGTM.

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

DIE *DwarfCompileUnit::constructVariableDIEImpl(const DbgVariable &DV,
 
   if (Abstract) {
     applyVariableAttributes(DV, *VariableDie);
     return VariableDie;
   }
 
   // Add variable address.
 
-  unsigned Offset = DV.getDebugLocListIndex();
-  if (Offset != ~0U) {
-    addLocationList(*VariableDie, dwarf::DW_AT_location, Offset);
+  unsigned Index = DV.getDebugLocListIndex();
+  if (Index != ~0U) {
+    addLocationList(*VariableDie, dwarf::DW_AT_location, Index);
     auto TagOffset = DV.getDebugLocListTagOffset();
     if (TagOffset)
       addUInt(*VariableDie, dwarf::DW_AT_LLVM_tag_offset, dwarf::DW_FORM_data1,
               *TagOffset);
     return VariableDie;
   }
 
   // Check if variable has a single location description.

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h

 /// indices.
 ///
 /// Variables can be created from \c DBG_VALUE instructions.  Those whose
 /// location changes over time use \a DebugLocListIndex, while those with a
 /// single location use \a ValueLoc and (optionally) a single entry of \a Expr.
 ///
 /// Variables that have been optimized out use none of these fields.
 class DbgVariable : public DbgEntity {
-  /// Offset in DebugLocs.
+  /// Index of the entry list in DebugLocs.
   unsigned DebugLocListIndex = ~0u;
   /// DW_OP_LLVM_tag_offset value from DebugLocs.
   Optional<uint8_t> DebugLocListTagOffset;
 
   /// Single value location description.
   std::unique_ptr<DbgValueLoc> ValueLoc = nullptr;
 
   struct FrameIndexExpr {