[llvm-exegesis] Fix r374158

Some bots complain about missing 'class':

LlvmState.h:70:40: error: declaration of ‘std::unique_ptr<const llvm::TargetMachine> llvm::exegesis::LLVMState::TargetMachine’ [-fpermissive]
   std::unique_ptr<const TargetMachine> TargetMachine;

llvm-svn: 374162