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

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

Please note: the guidelines do not require one to replace [] with at(), that's just one of the possible solutions. Actually at() is banned in many codebases where exceptions are banned.

It would be good to make this fix-it opt-in,  configurable via option, so the check only emits a warning by default.

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