[ThinLTO] Avoid archive member collisions with old API

[johanengelen]

Summary

ld64 on OSX uses the old ThinLTOCodegenerator API. When two modules have the same name in an archive (valid archive), a name collision happens for the modules' buffer identifiers.
This PR resolves this, by suffixing the module name with an increasing number such that the identifiers are guaranteed to be unique.

For a similar fix in LLD, see https://reviews.llvm.org/D25495

[johanengelen]

(I did not know how to test this well, so no test added)

[mehdi_amini]

LGTM.

Did you check ld64 in Xcode 9? I think ld64 was fixed to append the suffix as well.

[johanengelen]

I did not check Xcode 9 (can't on my current system :/ ).

lib/LTO/ThinLTOCodeGenerator.cpp

else if (TheTriple.getArch() == llvm::Triple::aarch64)
       TMBuilder.MCpu = "cyclone";
   }
   TMBuilder.TheTriple = std::move(TheTriple);
 }
 
 } // end anonymous namespace
 
 void ThinLTOCodeGenerator::addModule(StringRef Identifier, StringRef Data) {
-  ThinLTOBuffer Buffer(Data, Identifier);
+  std::string Id =
+      (Twine(Identifier) + "_" + std::to_string(Modules.size())).str();
+  ThinLTOBuffer Buffer(Data, std::move(Id));
   LLVMContext Context;
   StringRef TripleStr;
   ErrorOr<std::string> TripleOrErr = expectedToErrorOrAndEmitErrors(
       Context, getBitcodeTargetTriple(Buffer.getMemBuffer()));
 
   if (TripleOrErr)
     TripleStr = *TripleOrErr;