[clang-tools-extra] Enforce SL.con.3: Add check to replace operator[] with at() [Cont.] (PR #95220)

[Paul Heidekrüger via cfe-commits]

paulhdk wrote:

> This looks basically done, thanks for sticking with it

Thanks so much for taking another look, @5chmidti!
I ran into some questions; see the comments I left above.

https://github.com/llvm/llvm-project/pull/95220