Upgrade clang-tidy on CI. (#5469)

* Correct all clang-tidy errors.
* Upgrade clang-tidy to 10 on CI.

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

tests/cpp/test_learner.cc

@@ -86,7 +86,7 @@ TEST(Learner, CheckGroup) {
   EXPECT_ANY_THROW(learner->UpdateOneIter(0, p_mat));
 }
 
-TEST(Learner, SLOW_CheckMultiBatch) {
+TEST(Learner, SLOW_CheckMultiBatch) {  // NOLINT
   // Create sufficiently large data to make two row pages
   dmlc::TemporaryDirectory tempdir;
   const std::string tmp_file = tempdir.path + "/big.libsvm";