Fix Mem leak in LLVMTargetMachine.cpp

[XinWang10]

Summary

If we go to line 302, with one of MCE or MAB is not nullptr, then we could
leak mem here.
Use unique_ptr to maintain these 2 pointer can avoid it.

[XinWang10]

• use unique_ptr instead

[LuoYuanke]

LGTM

llvm/lib/CodeGen/LLVMTargetMachine.cpp

bool LLVMTargetMachine::addPassesToEmitMC(PassManagerBase &PM, MCContext *&Ctx,
   Options.MCOptions.EmitDwarfUnwind = EmitDwarfUnwindType::Always;
   if (Options.MCOptions.MCSaveTempLabels)
     Ctx->setAllowTemporaryLabels(false);
 
   // Create the code emitter for the target if it exists.  If not, .o file
   // emission fails.
   const MCSubtargetInfo &STI = *getMCSubtargetInfo();
   const MCRegisterInfo &MRI = *getMCRegisterInfo();
-  MCCodeEmitter *MCE = getTarget().createMCCodeEmitter(*getMCInstrInfo(), *Ctx);
-  MCAsmBackend *MAB =
-      getTarget().createMCAsmBackend(STI, MRI, Options.MCOptions);
+  std::unique_ptr<MCCodeEmitter> MCE(
+      getTarget().createMCCodeEmitter(*getMCInstrInfo(), *Ctx));
+  std::unique_ptr<MCAsmBackend> MAB(
+      getTarget().createMCAsmBackend(STI, MRI, Options.MCOptions));
   if (!MCE || !MAB)
     return true;
 
   const Triple &T = getTargetTriple();
   std::unique_ptr<MCStreamer> AsmStreamer(getTarget().createMCObjectStreamer(
-      T, *Ctx, std::unique_ptr<MCAsmBackend>(MAB), MAB->createObjectWriter(Out),
-      std::unique_ptr<MCCodeEmitter>(MCE), STI, Options.MCOptions.MCRelaxAll,
+      T, *Ctx, std::move(MAB), MAB->createObjectWriter(Out), std::move(MCE),
+      STI, Options.MCOptions.MCRelaxAll,
       Options.MCOptions.MCIncrementalLinkerCompatible,
       /*DWARFMustBeAtTheEnd*/ true));
 
   // Create the AsmPrinter, which takes ownership of AsmStreamer if successful.
   FunctionPass *Printer =
       getTarget().createAsmPrinter(*this, std::move(AsmStreamer));
   if (!Printer)
     return true;
 
   PM.add(Printer);
   PM.add(createFreeMachineFunctionPass());
 
   return false; // success!
 }