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

[Paul Heidekrüger via cfe-commits]

PBHDK wrote:

> I am adding @leunam99 and @PBHDK to the PR, who will contribute in the next few weeks.

Since we cannot push to this PR, we will be submitting a new PR with @sebwolf-de's + our work, correct, @EugeneZelenko, @PiotrZSL, @HerrCai0907?

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