[ELF] - Return file offset as address only for allocatable sections when building .gdb_index

[grimar]

Summary

It is my suggestion for fixing issue we discussing in
"[llvm] r289961 - Revert "dwarfdump: Support/process relocations on a CU's abbrev_off"" thread.

Since callback named getSectionLoadAddress() it looks reasonable to make it return address only
for allocatable sections. That works for LLD needs and solves problem.

[aprantl]

ELF/GdbIndex.cpp

190

Would returning an Optional<uint64_t> be better here?

ELF/GdbIndex.cpp

for (std::pair<uint64_t, uint64_t> &R : Ranges)
       if (InputSectionBase<ELFT> *S = findSection(Sections, R.first))
         Ret.push_back(
             {S, R.first - S->Offset, R.second - S->Offset, CurrentCU});
     ++CurrentCU;
   }
   return Ret;
 }
 
+// We return file offset as load address for allocatable sections. That is
+// currently used for collecting address ranges in readAddressArea(). We are
+// able then to find section index that range belongs to.
 template <class ELFT>
 uint64_t GdbIndexBuilder<ELFT>::getSectionLoadAddress(
     const object::SectionRef &Sec) const {
+  if (static_cast<const ELFSectionRef &>(Sec).getFlags() & ELF::SHF_ALLOC)
     return static_cast<const ELFSectionRef &>(Sec).getOffset();
+  return 0;
 }
 
 template <class ELFT>
 std::unique_ptr<LoadedObjectInfo> GdbIndexBuilder<ELFT>::clone() const {
   return {};
 }
 
 namespace lld {
 namespace elf {
 template class GdbIndexBuilder<ELF32LE>;
 template class GdbIndexBuilder<ELF32BE>;
 template class GdbIndexBuilder<ELF64LE>;
 template class GdbIndexBuilder<ELF64BE>;
 }
 }