Use object address instead of offset in an archive file.

[ruiu]

Summary

This patch is more like a question than a real change I want to submit,
but can we just use object address to make each file in an archive has
a unique identifier? If this is OK, we don't need to pass OffsetInArchive
to the ctor.

[pcc]

I believe that these names can end up in various observable places including -save-temps file names and combined module summary indexes, so I don't think we can do this.

lld/ELF/InputFiles.cpp

BitcodeFile::BitcodeFile(MemoryBufferRef MB, StringRef ArchiveName,
   // 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).
   MemoryBufferRef MBRef(
       MB.getBuffer(),
       Saver.save(ArchiveName + MB.getBufferIdentifier() +
-                 (ArchiveName.empty() ? "" : utostr(OffsetInArchive))));
+                 (ArchiveName.empty() ? "" : utostr(reinterpret_cast<uint64_t>(MB.getBufferStart())))));
   Obj = CHECK(lto::InputFile::create(MBRef), this);
 
   Triple T(Obj->getTargetTriple());
   EKind = getBitcodeELFKind(T);
   EMachine = getBitcodeMachineKind(MB.getBufferIdentifier(), T);
 }
 
 static uint8_t mapVisibility(GlobalValue::VisibilityTypes GvVisibility) {