[clang-tools-extra] [clang-tidy] [NFC] Move comment scanning to `LexerUtils` and add tests (PR #180371)

[Victor Chernyakin via cfe-commits]

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

One review is alright for a pretty simple change like this one, but having 2 or more is best ^_^

https://github.com/llvm/llvm-project/pull/180371