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

[Congcong Cai via cfe-commits]

HerrCai0907 wrote:

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

I don't run clang-tidy bench for some large project. But I do some test for some auto-generated cpp file which has lots of using, and the execution time reduce from 0.2s to 0.18s in my MAC M1 Pro.

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