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

[Congcong Cai via cfe-commits]

https://github.com/HerrCai0907 requested changes to this pull request.

`at` and `operator[]` in `std::map` have different meanings. `operator[]` will insert value if key does not exist but `at` won't. Please limit this check only for index containers instead of key-value containers.

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