[clang] [Wunsafe-buffer-usage] Address some false positives in handling array indices that are decidably correct (PR #117370)

[Reid Kleckner via cfe-commits]

https://github.com/rnk commented:

I think there's a policy question here of how much trust we want to put into the type system. Clearly, we've already put some trust into it to reduce false positives, but we could decide to trust any old constant array type bounds, and that would be a good code simplification.

https://github.com/llvm/llvm-project/pull/117370