Use Default trait for Position and DisplayFunctionAnnotations (#843)

cranelift/frontend/src/frontend.rs

@@ -46,6 +46,7 @@ struct EbbData {
     user_param_count: usize,
 }
 
+#[derive(Default)]
 struct Position {
     ebb: PackedOption<Ebb>,
     basic_block: PackedOption<Block>,
@@ -59,13 +60,6 @@ impl Position {
         }
     }
 
-    fn default() -> Self {
-        Self {
-            ebb: PackedOption::default(),
-            basic_block: PackedOption::default(),
-        }
-    }
-
     fn is_default(&self) -> bool {
         self.ebb.is_none() && self.basic_block.is_none()
     }