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

[Baranov Victor via cfe-commits]

vbvictor wrote:

At least 2 reviews are most likely need for new checks, for bugfixes 1 can be enough if no one else wants to review. 
As a rule of thumb, wait a couple of days after first approval and then land of no one else reviews in that period.

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