[clang-tools-extra] Enforce SL.con.3: Add check to replace operator[] with at() (PR #90043)

[Piotr Zegar via cfe-commits]

https://github.com/PiotrZSL requested changes to this pull request.

Idea is fine, implementation is lacking critical parts.
Please check comments, fix them.
If you will have some questions, just ask.

https://github.com/llvm/llvm-project/pull/90043