[clang-tools-extra] [clang-tidy] Add llvm-regex check (PR #207407)

[Donát Nagy via cfe-commits]

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

At first glance LGTM, except for a typo in the example code in the documentation (marked with inline comment).

https://github.com/llvm/llvm-project/pull/207407