[ELF] Use SymbolTable::ComdatGroups instead of DummyGroups in LTO

[ruiu]

That doesn't work -- you always want to add symbols again after LTO to override existing bitcode symbols with real symbols, so you don't want to guard bitcode symbols with the same set of strings.

[pcc]

Yes, unless you're guaranteed that none of the object files will contain any comdats (and I don't believe that the backend makes anything near this guarantee right now), you're going to end up dropping "real" definitions on the floor with this.

[MaskRay]

Use ruiu@'s version

[MaskRay]

Abandon since ruiu wants to work on this in D56015

ELF/SymbolTable.cpp

template <class ELFT> void SymbolTable::addCombinedLTOObject() {
   if (BitcodeFiles.empty())
     return;
 
   // Compile bitcode files and replace bitcode symbols.
   LTO.reset(new BitcodeCompiler);
   for (BitcodeFile *F : BitcodeFiles)
     LTO->add(*F);
 
-  for (InputFile *File : LTO->compile()) {
-    DenseSet<CachedHashStringRef> DummyGroups;
+  for (InputFile *File : LTO->compile()) {
     auto *Obj = cast<ObjFile<ELFT>>(File);
     Obj->parse(DummyGroups);
     for (Symbol *Sym : Obj->getGlobalSymbols())
       Sym->parseSymbolVersion();
     ObjectFiles.push_back(File);
   }
 }