Rename node_index as index.

For consistency with other variables in this file.

src/librustc_data_structures/obligation_forest/mod.rs

@@ -373,8 +373,8 @@ pub fn map_pending_obligations<P, F>(&self, f: F) -> Vec<P>
             .collect()
     }
 
-    fn insert_into_error_cache(&mut self, node_index: usize) {
-        let node = &self.nodes[node_index];
+    fn insert_into_error_cache(&mut self, index: usize) {
+        let node = &self.nodes[index];
         self.error_cache
             .entry(node.obligation_tree_id)
             .or_default()