[clang] [clang] Emit nuw GEPs for array subscript expressions (PR #103088)

[Hari Limaye via cfe-commits]

hazzlim wrote:

This seems correct from my reading of the relevant parts of the C/C++ standard, regarding pointer arithmetic and array subscript expressions.

It does actually seem like the restriction to unsigned indices here is actually unnecessary, and adding `nuw` is also valid for signed indices here - but I wanted to check if people agree on that.

https://github.com/llvm/llvm-project/pull/103088