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

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

To be clear and reiterate my previous comment: this check should NOT require users to use at(). That behavior should be opt-in. It should only warn about using operator[]. It's up to the users to figure out what the best replacement is.

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