ELF: Remove field for .gdb_index in InStruct. NFC.

[MaskRay]

Summary

This field is unreferenced outside of createSyntheticSections.

[grimar]

LGTM

[MaskRay]

Rebase

ELF/SyntheticSections.h

struct InStruct {
   EhFrameHeader *EhFrameHdr;
   EhFrameSection *EhFrame;
   SyntheticSection *Dynamic;
   StringTableSection *DynStrTab;
   SymbolTableBaseSection *DynSymTab;
   GnuHashTableSection *GnuHashTab;
   HashTableSection *HashTab;
   InputSection *Interp;
-  GdbIndexSection *GdbIndex;
   GotSection *Got;
   GotPltSection *GotPlt;
   IgotPltSection *IgotPlt;
   PPC64LongBranchTargetSection *PPC64LongBranchTarget;
   MipsGotSection *MipsGot;
   MipsRldMapSection *MipsRldMap;
   PltSection *Plt;
   PltSection *Iplt;

ELF/Writer.cpp

if (Config->EMachine == EM_PPC64) {
     Add(In.PPC64LongBranchTarget);
   }
 
   In.GotPlt = make<GotPltSection>();
   Add(In.GotPlt);
   In.IgotPlt = make<IgotPltSection>();
   Add(In.IgotPlt);
 
-  if (Config->GdbIndex) {
-    In.GdbIndex = GdbIndexSection::create<ELFT>();
-    Add(In.GdbIndex);
-  }
+  if (Config->GdbIndex)
+    Add(GdbIndexSection::create<ELFT>());
 
   // We always need to add rel[a].plt to output if it has entries.
   // Even for static linking it can contain R_[*]_IRELATIVE relocations.
   In.RelaPlt = make<RelocationSection<ELFT>>(
       Config->IsRela ? ".rela.plt" : ".rel.plt", false /*Sort*/);
   Add(In.RelaPlt);
 
   // The RelaIplt immediately follows .rel.plt (.rel.dyn for ARM) to ensure