"Cachify" ExternPreludeEntry.binding through a Cell.

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -984,18 +984,17 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                 // more details: https://github.com/rust-lang/rust/pull/111761
                 return;
             }
-            let entry = self
-                .r
-                .extern_prelude
-                .entry(ident)
-                .or_insert(ExternPreludeEntry { binding: None, introduced_by_item: true });
+            let entry = self.r.extern_prelude.entry(ident).or_insert(ExternPreludeEntry {
+                binding: Cell::new(None),
+                introduced_by_item: true,
+            });
             if orig_name.is_some() {
                 entry.introduced_by_item = true;
             }
             // Binding from `extern crate` item in source code can replace
             // a binding from `--extern` on command line here.
             if !entry.is_import() {
-                entry.binding = Some(imported_binding)
+                entry.binding.set(Some(imported_binding));
             } else if ident.name != kw::Underscore {
                 self.r.dcx().span_delayed_bug(
                     item.span,