[llvm] [WIP][IR][Constants] Change the semantic of `ConstantPointerNull` to represent an actual `nullptr` instead of a zero-value pointer (PR #166667)

[Alexander Richardson via llvm-commits]

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@@ -193,9 +193,11 @@ constexpr DataLayout::PrimitiveSpec DefaultVectorSpecs[] = {
 };
 
 // Default pointer type specifications.
-constexpr DataLayout::PointerSpec DefaultPointerSpecs[] = {
+const DataLayout::PointerSpec DefaultPointerSpecs[] = {
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arichardson wrote:

We could avoid the need for a global ctor/dtor if we just stored a plain integer value? Since we only support 0/-1 we could just rely on sign-extension? But of course that makes the getter slower so probably best to ignore this suggestion.

https://github.com/llvm/llvm-project/pull/166667