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

[Baranov Victor via cfe-commits]

vbvictor wrote:

I'm in the process of looking at old/duplicate PRs to lower number of ongoing open requests (currently 87) so closing would be helpful.

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