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`

compiler/rustc_ast/src/ast.rs

@@ -2036,6 +2036,14 @@ impl TyKind {
     pub fn is_unit(&self) -> bool {
         matches!(self, TyKind::Tup(tys) if tys.is_empty())
     }
+
+    pub fn is_simple_path(&self) -> Option<Symbol> {
+        if let TyKind::Path(None, Path { segments, .. }) = &self && segments.len() == 1 {
+            Some(segments[0].ident.name)
+        } else {
+            None
+        }
+    }
 }
 
 /// Syntax used to declare a trait object.