Use nullptr instead of NULL for variadic sentinels

Windows defines NULL to 0, which when used as an argument to a variadic
function, is not a null pointer constant. As a result, Clang's
-Wsentinel fires on this code. Using '0' would be wrong on most 64-bit
platforms, but both MSVC and Clang make it work on Windows. Sidestep the
issue with nullptr.

llvm-svn: 221940

llvm/lib/CodeGen/StackProtector.cpp

@@ -480,12 +480,12 @@ BasicBlock *StackProtector::CreateFailBB() {
   if (Trip.getOS() == llvm::Triple::OpenBSD) {
     Constant *StackChkFail = M->getOrInsertFunction(
         "__stack_smash_handler", Type::getVoidTy(Context),
-        Type::getInt8PtrTy(Context), NULL);
+        Type::getInt8PtrTy(Context), nullptr);
 
     B.CreateCall(StackChkFail, B.CreateGlobalStringPtr(F->getName(), "SSH"));
   } else {
     Constant *StackChkFail = M->getOrInsertFunction(
-        "__stack_chk_fail", Type::getVoidTy(Context), NULL);
+        "__stack_chk_fail", Type::getVoidTy(Context), nullptr);
     B.CreateCall(StackChkFail);
   }
   B.CreateUnreachable();