[clang-tools-extra] [NFC][clang-tidy]improve performance for misc-unused-using-decls check (PR #78231)

[Haojian Wu via cfe-commits]

https://github.com/hokein approved this pull request.

Thanks, the code looks good. Do you have any performance data after this change?

https://github.com/llvm/llvm-project/pull/78231