[Bugpoint] Erase comdat annotat after setting linkage to external.

[bryant]

Summary

Not entirely sure whether this is correct. I was unable to use Bugpoint to debug a miscompilation because its function splitting phase caused many verifier errors of this type: "Declaration may not be in a Comdat!" This patch resolved the issue.

This is similar in spirit to https://reviews.llvm.org/D21274 .

[rnk]

Why not do this in or after DeleteFunctionBody? That's when it becomes a declaration.

tools/bugpoint/ExtractFunction.cpp

static void SplitStaticCtorDtor(const char *GlobalName, Module *M1, Module *M2,
   }
 }
 
 std::unique_ptr<Module>
 llvm::SplitFunctionsOutOfModule(Module *M, const std::vector<Function *> &F,
                                 ValueToValueMapTy &VMap) {
   // Make sure functions & globals are all external so that linkage
   // between the two modules will work.
-  for (Module::iterator I = M->begin(), E = M->end(); I != E; ++I)
+  for (Module::iterator I = M->begin(), E = M->end(); I != E; ++I) {
     I->setLinkage(GlobalValue::ExternalLinkage);
+    I->setComdat(nullptr);
+  }
   for (Module::global_iterator I = M->global_begin(), E = M->global_end();
        I != E; ++I) {
     if (I->hasName() && I->getName()[0] == '\01')
       I->setName(I->getName().substr(1));
     I->setLinkage(GlobalValue::ExternalLinkage);
+    I->setComdat(nullptr);
   }
 
   ValueToValueMapTy NewVMap;
   std::unique_ptr<Module> New = CloneModule(M, NewVMap);
 
   // Remove the Test functions from the Safe module
   std::set<Function *> TestFunctions;
   for (unsigned i = 0, e = F.size(); i != e; ++i) {