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

[Nikita Popov via llvm-commits]

nikic wrote:

Which targets are going to use `No`?

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I assume that the address of the null pointer is going to be considered non-dereferenceable (once this work is complete) unless null_pointer_is_valid is used? That is, the special rule that 0 pointer is dereferenceable in non-default AS will go away? And the default is going to be that 0 is non-dereferenceable in all address spaces by dint of `Nz` being the default?

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