Fix clang-tidy error

Enhancement

There are still a lot of clang-tidy error in our code. In order to improve code quality, we need to fix them.

A script to detect clang-tidy error in local machine using clang-tidy:

# find_clang_tidy_error.sh
## env settings
WORKSPACE=$(cd `dirname $0`; pwd)
BUILD_DIR="build"
TIDY_BIN="/usr/bin/clang-tidy"
TIDY_SCRIPT="/path/to/tiflash/release-centos7-llvm/scripts/run-clang-tidy.py"

DIR_TO_CHECK="${1}"

command -v "${TIDY_SCRIPT}" > /dev/null 2>&1
if [[ $? == 0 ]]; then
  # Check files by script, with parallel
  python3 "${TIDY_SCRIPT}" -p "${WORKSPACE}/${BUILD_DIR}/" -j 4 --files "${DIR_TO_CHECK}/*"
else
  # Check each file by clang-tidy
  for f in $(find "${DIR_TO_CHECK}" -name '*.cpp' -o -name "*.h"); do
    echo "Checking $f"
      "${TIDY_BIN}" -p "${WORKSPACE}/${BUILD_DIR}/compile_commands.json" "$f"
  done
fi

how to run:

./find_clang_tidy_error.sh dbms/src/Storages/DeltaMerge/tests

Be free to open a new issue like #4602 #4603 #4604, and a linked PR is warmly welcomed!

note that clang-tidy is just a tool for static analysis, so the result of clang-tidy is not absolutely right.

Start by using the provided find_clang_tidy_error.sh command with clang-tidy on a selected directory, such as dbms/src/Storages/DeltaMerge/tests. Review the reported diagnostics and address an appropriate subset of them, then rerun the command to confirm those diagnostics are resolved.