OptBisect.cpp - remove unused include. NFCI.

StringRef.h is included in OptBisect.h and we have no uses of std::string.

llvm/lib/IR/OptBisect.cpp

@@ -13,13 +13,11 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/IR/OptBisect.h"
-#include "llvm/ADT/StringRef.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <limits>
-#include <string>
 
 using namespace llvm;