[llvm] [RFC][DataLayout] Add null pointer value infrastructure (PR #183207)

[Nikita Popov via llvm-commits]

nikic wrote:

> Is there any value in adding `u` for "unknown", meaning an arbitrary bit pattern can represent `nullptr`, to leave the door open for future targets that might use something like `0x7f7f7f7f7f7f`? In that case, LLVM wouldn't fold based on it, but also wouldn't restrict the null pointer value to all zeros or all ones.

I'd be fine with having that, but I also don't think we need to preemptively add it. Once the rest is in place, I think this would be quite easy to retrofit if we find a use case for it.

https://github.com/llvm/llvm-project/pull/183207