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

[Piotr Zegar via cfe-commits]

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

LGTM.
There is open issue for performance of this check.
Consider mentioning this in release notes as some generic performance improvment.
Do you have any benchmarks ?


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