Auto merge of #94527 - oli-obk:undef_scalars, r=nagisa,erikdesjardin

Let CTFE to handle partially uninitialized unions without marking the entire value as uninitialized.

follow up to #94411

To fix https://github.com/rust-lang/rust/issues/69488 and by extension fix https://github.com/rust-lang/rust/issues/94371, we should stop treating types like `MaybeUninit<usize>` as something that the `Scalar` type in the interpreter engine can represent. So we add a new field to `abi::Primitive` that records whether the primitive is nested in a union

cc `@RalfJung`

r? `@ghost`

src/librustdoc/html/render/print_item.rs

@@ -1769,7 +1769,7 @@ fn document_type_layout(w: &mut Buffer, cx: &Context<'_>, ty_def_id: DefId) {
 
                     let tag_size = if let TagEncoding::Niche { .. } = tag_encoding {
                         0
-                    } else if let Primitive::Int(i, _) = tag.value {
+                    } else if let Primitive::Int(i, _) = tag.primitive() {
                         i.size().bytes()
                     } else {
                         span_bug!(tcx.def_span(ty_def_id), "tag is neither niche nor int")