[clang] [CIR] Upstream pointer subtraction handling (PR #163306)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> The code gen checks were made with LLM assistance. I apologize if they look weird or need to be fixed, I am not familiar with IR.

Thanks for disclosing that. It gives important context for reviewing. I often use AI to generate checks myself, but they generally need a bit of fixup afterwards. It does save a lot of time though, so I think it's a very good starting point.

https://github.com/llvm/llvm-project/pull/163306