COFF: Destroy LTOModules as they are linked.

[pcc]

Summary

This should help reduce memory consumption during LTO. Also works around a
bug [1] in the module linker.

[1] http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20151109/312869.html

[ruiu]

LGTM

lld/trunk/COFF/InputFiles.h

 // Used for LTO.
 class BitcodeFile : public InputFile {
 public:
   explicit BitcodeFile(MemoryBufferRef M) : InputFile(BitcodeKind, M) {}
   static bool classof(const InputFile *F) { return F->kind() == BitcodeKind; }
   std::vector<SymbolBody *> &getSymbols() override { return SymbolBodies; }
   MachineTypes getMachineType() override;
 
-  LTOModule *getModule() const { return M.get(); }
   std::unique_ptr<LTOModule> takeModule() { return std::move(M); }
 
 private:
   void parse() override;
 
   std::vector<SymbolBody *> SymbolBodies;
   llvm::BumpPtrAllocator Alloc;
   std::unique_ptr<LTOModule> M;
   static std::mutex Mu;
 };
 
 } // namespace coff
 } // namespace lld
 
 #endif

lld/trunk/COFF/SymbolTable.cpp

for (Undefined *U : Config->GCRoot) {
     if (auto *S = dyn_cast<DefinedBitcode>(U->repl()))
       CG->addMustPreserveSymbol(S->getName());
     else if (auto *S = dyn_cast_or_null<DefinedBitcode>(U->getWeakAlias()))
       CG->addMustPreserveSymbol(S->getName());
   }
 
   CG->setModule(BitcodeFiles[0]->takeModule());
   for (unsigned I = 1, E = BitcodeFiles.size(); I != E; ++I)
-    CG->addModule(BitcodeFiles[I]->getModule());
+    CG->addModule(BitcodeFiles[I]->takeModule().get());
 
   bool DisableVerify = true;
 #ifdef NDEBUG
   DisableVerify = false;
 #endif
   if (!CG->optimize(DisableVerify, false, false, false))
     error(""); // optimize() should have emitted any error message.