Replace a few instances of NULL with nullptr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230599 91177308-0d34-0410-b5e6-96231b3b80d8

tools/lli/lli.cpp

@@ -597,7 +597,7 @@ int main(int argc, char **argv, char * const *envp) {
     // function later on to make an explicit call, so get the function now.
     Constant *Exit = Mod->getOrInsertFunction("exit", Type::getVoidTy(Context),
                                                       Type::getInt32Ty(Context),
-                                                      NULL);
+                                                      nullptr);
 
     // Run static constructors.
     if (!ForceInterpreter) {