[clang] [Clang][CodeGen] Check `isUnderlyingBasePointerConstantNull` in `emitPointerArithmetic` (PR #137849)

[Yingwei Zheng via cfe-commits]

dtcxzyw wrote:

> While we should omit inbounds, I think it would be good to not suppress the sanitizer. If the sanitizer doesn't warn about it, then things will never get better :(

Fixed.

https://github.com/llvm/llvm-project/pull/137849