Update lld to use new LTO comdat api

[davide]

I'll wait until the related patch goes in to review this one, but it should be a pretty mechanical change.

[davide]

Now that D25697 is unblocked, this LGTM too. Thanks for the time you took improving the API =)

ELF/InputFiles.cpp

case GlobalValue::HiddenVisibility:
     return STV_HIDDEN;
   case GlobalValue::ProtectedVisibility:
     return STV_PROTECTED;
   }
   llvm_unreachable("unknown visibility");
 }
 
 template <class ELFT>
-static Symbol *createBitcodeSymbol(DenseSet<CachedHashStringRef> &KeptComdats,
-                                   DenseSet<CachedHashStringRef> &ComdatGroups,
+static Symbol *createBitcodeSymbol(const std::vector<bool> &KeptComdats,
                                    const lto::InputFile::Symbol &ObjSym,
                                    StringSaver &Saver, BitcodeFile *F) {
   StringRef NameRef = Saver.save(ObjSym.getName());
   uint32_t Flags = ObjSym.getFlags();
   uint32_t Binding = (Flags & BasicSymbolRef::SF_Weak) ? STB_WEAK : STB_GLOBAL;
 
   uint8_t Type = ObjSym.isTLS() ? STT_TLS : STT_NOTYPE;
   uint8_t Visibility = mapVisibility(ObjSym.getVisibility());
   bool CanOmitFromDynSym = ObjSym.canBeOmittedFromSymbolTable();
 
-  StringRef C = check(ObjSym.getComdat());
-  if (!C.empty()) {
-    auto CH = CachedHashStringRef(C);
-    bool Keep = KeptComdats.count(CH);
-    if (!Keep) {
-      StringRef N = Saver.save(C);
-      CachedHashStringRef NH(N, CH.hash());
-      if (ComdatGroups.insert(NH).second) {
-        Keep = true;
-        KeptComdats.insert(NH);
-      }
-    }
-    if (!Keep)
-      return Symtab<ELFT>::X->addUndefined(NameRef, Binding, Visibility, Type,
-                                           CanOmitFromDynSym, F);
-  }
+  int C = check(ObjSym.getComdatIndex());
+  if (C != -1 && !KeptComdats[C])
 
   if (Flags & BasicSymbolRef::SF_Undefined)
     return Symtab<ELFT>::X->addUndefined(NameRef, Binding, Visibility, Type,
                                          CanOmitFromDynSym, F);
 
   if (Flags & BasicSymbolRef::SF_Common)
     return Symtab<ELFT>::X->addCommon(NameRef, ObjSym.getCommonSize(),
                                       ObjSym.getCommonAlignment(), Binding,
void BitcodeFile::parse(DenseSet<CachedHashStringRef> &ComdatGroups) {
   // ThinLTO uses the MemoryBufferRef identifier to access its internal
   // data structures and if two archives define two members with the same name,
   // this causes a collision which result in only one of the objects being
   // taken into consideration at LTO time (which very likely causes undefined
   // symbols later in the link stage).
   Obj = check(lto::InputFile::create(MemoryBufferRef(
       MB.getBuffer(), Saver.save(ArchiveName + MB.getBufferIdentifier() +
                                  utostr(OffsetInArchive)))));
-  DenseSet<CachedHashStringRef> KeptComdats;
+
+  std::vector<bool> KeptComdats;
+  for (StringRef S : Obj->getComdatTable()) {
+    StringRef N = Saver.save(S);
+    KeptComdats.push_back(ComdatGroups.insert(CachedHashStringRef(N)).second);
+  }
+
   for (const lto::InputFile::Symbol &ObjSym : Obj->symbols())
-    Symbols.push_back(createBitcodeSymbol<ELFT>(KeptComdats, ComdatGroups,
-                                                ObjSym, Saver, this));
+    Symbols.push_back(
+        createBitcodeSymbol<ELFT>(KeptComdats, ObjSym, Saver, this));
 }
 
 template <template <class> class T>
 static InputFile *createELFFile(MemoryBufferRef MB) {
   unsigned char Size;
   unsigned char Endian;
   std::tie(Size, Endian) = getElfArchType(MB.getBuffer());
   if (Endian != ELFDATA2LSB && Endian != ELFDATA2MSB)