Auto merge of #98758 - nnethercote:more-derive-output-improvements, r=Mark-Simulacrum

More derive output improvements

This PR includes:
- Some test improvements.
- Some cosmetic changes to derive output that make the code look more like what a human would write.
- Some more fundamental improvements to `cmp` and `partial_cmp` generation.

r? `@Mark-Simulacrum`

src/test/ui/deriving/deriving-all-codegen.rs

@@ -39,6 +39,16 @@ struct Big {
 #[repr(packed)]
 struct Packed(u32);
 
+// An empty enum.
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+enum Enum0 {}
+
+// A single-variant enum.
+#[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+enum Enum1 {
+    Single { x: u32 }
+}
+
 // A C-like, fieldless enum.
 #[derive(Clone, Copy, Debug, Default, Hash, PartialEq, Eq, PartialOrd, Ord)]
 enum Fieldless {
@@ -66,3 +76,11 @@ enum Fielded {
     Y(bool),
     Z(Option<i32>),
 }
+
+// A union. Most builtin traits are not derivable for unions.
+#[derive(Clone, Copy)]
+pub union Union {
+    pub b: bool,
+    pub u: u32,
+    pub i: i32,
+}