Reduce display_map snapshot creation (#39354)

Re-applies https://github.com/zed-industries/zed/pull/30840

This PR re-applies the initial
[PR](https://github.com/zed-industries/zed/pull/30840). As it was closed
because it was hard to land, because of the many conflicts. This PR
re-applies the changes for it.

In several cases we were creating multiple display_map
snapshots within the same root-level function call.
Creating a display_map snapshot is quite slow, and in some
cases we were creating the snapshot multiple times.

Release Notes:

- N/A

crates/diagnostics/src/items.rs

@@ -170,7 +170,10 @@ impl DiagnosticIndicator {
     fn update(&mut self, editor: Entity<Editor>, window: &mut Window, cx: &mut Context<Self>) {
         let (buffer, cursor_position) = editor.update(cx, |editor, cx| {
             let buffer = editor.buffer().read(cx).snapshot(cx);
-            let cursor_position = editor.selections.newest::<usize>(cx).head();
+            let cursor_position = editor
+                .selections
+                .newest::<usize>(&editor.display_snapshot(cx))
+                .head();
             (buffer, cursor_position)
         });
         let new_diagnostic = buffer