Detect more `cfg`d out items in resolution errors

Use a visitor to collect *all* items (including those nested) that were stripped behind a `cfg` condition.

```
error[E0425]: cannot find function `f` in this scope
  --> $DIR/nested-cfg-attrs.rs:4:13
   |
LL | fn main() { f() }
   |             ^ not found in this scope
   |
note: found an item that was configured out
  --> $DIR/nested-cfg-attrs.rs:2:4
   |
LL | fn f() {}
   |    ^
note: the item is gated here
  --> $DIR/nested-cfg-attrs.rs:1:35
   |
LL | #[cfg_attr(all(), cfg_attr(all(), cfg(FALSE)))]
   |                                   ^^^^^^^^^^
```

1 files changed, 2 insertions, 3 deletions

diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index 98e48664e68..236b1404eeb 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -525,9 +525,8 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         }
         self.err_code_special_cases(&mut err, source, path, span);
 
-        if let Some(module) = base_error.module {
-            self.r.find_cfg_stripped(&mut err, &path.last().unwrap().ident.name, module);
-        }
+        let module = base_error.module.unwrap_or_else(|| CRATE_DEF_ID.to_def_id());
+        self.r.find_cfg_stripped(&mut err, &path.last().unwrap().ident.name, module);
 
         (err, candidates)
     }