MetadataLoader: simplify old type ref upgrade by removing a redundant set (NFC)

[mehdi_amini]

Summary

I'm still trying to understand the logic, hopefully this is correct
(at least the validation is still passing).

[dexonsmith]

I think we discussed this in person at the time, and my understanding of metadata-loading is now a bit hazy, but IIRC this was a performance optimization. IOW, I think removing this would be valid, but would be a major performance regression when linking in old bitcode files that have debug info in them.

llvm/lib/Bitcode/Reader/MetadataLoader.cpp

class BitcodeReaderMetadataList {
   /// Don't use std::vector here.  Some versions of libc++ copy (instead of
   /// move) on resize, and TrackingMDRef is very expensive to copy.
   SmallVector<TrackingMDRef, 1> MetadataPtrs;
 
   /// Structures for resolving old type refs.
   struct {
     SmallDenseMap<MDString *, TempMDTuple, 1> Unknown;
     SmallDenseMap<MDString *, DICompositeType *, 1> Final;
-    SmallDenseMap<MDString *, DICompositeType *, 1> FwdDecls;
     SmallVector<std::pair<TrackingMDRef, TempMDTuple>, 1> Arrays;
   } OldTypeRefs;
 
   LLVMContext &Context;
 
 public:
   BitcodeReaderMetadataList(LLVMContext &C) : Context(C) {}
 

 
 void BitcodeReaderMetadataList::tryToResolveCycles() {
   if (NumFwdRefs)
     // Still forward references... can't resolve cycles.
     return;
 
   bool DidReplaceTypeRefs = false;
 
-  // Give up on finding a full definition for any forward decls that remain.
-  for (const auto &Ref : OldTypeRefs.FwdDecls)
-    OldTypeRefs.Final.insert(Ref);
-  OldTypeRefs.FwdDecls.clear();
-
   // Upgrade from old type ref arrays.  In strange cases, this could add to
   // OldTypeRefs.Unknown.
   for (const auto &Array : OldTypeRefs.Arrays) {
     DidReplaceTypeRefs = true;
     Array.second->replaceAllUsesWith(resolveTypeRefArray(Array.first.get()));
   }
   OldTypeRefs.Arrays.clear();
 
void BitcodeReaderMetadataList::tryToResolveCycles() {
   AnyFwdRefs = false;
   MinFwdRef = 0;
   MaxFwdRef = 0;
 }
 
 void BitcodeReaderMetadataList::addTypeRef(MDString &UUID,
                                            DICompositeType &CT) {
   assert(CT.getRawIdentifier() == &UUID && "Mismatched UUID");
-  if (CT.isForwardDecl())
-    OldTypeRefs.FwdDecls.insert(std::make_pair(&UUID, &CT));
-  else
-    OldTypeRefs.Final.insert(std::make_pair(&UUID, &CT));
 }
 
 Metadata *BitcodeReaderMetadataList::upgradeTypeRef(Metadata *MaybeUUID) {
   auto *UUID = dyn_cast_or_null<MDString>(MaybeUUID);
   if (LLVM_LIKELY(!UUID))
     return MaybeUUID;
 
   if (auto *CT = OldTypeRefs.Final.lookup(UUID))