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, 4 insertions, 3 deletions

diff --git a/tests/ui/rustdoc/cfg-rustdoc.rs b/tests/ui/rustdoc/cfg-rustdoc.rs
index dd8e1ed97c4..11c48d07552 100644
--- a/tests/ui/rustdoc/cfg-rustdoc.rs
+++ b/tests/ui/rustdoc/cfg-rustdoc.rs
@@ -1,6 +1,7 @@
-#[cfg(doc)]
-pub struct Foo;
+#[cfg(doc)] //~ NOTE the item is gated here
+pub struct Foo; //~ NOTE found an item that was configured out
 
 fn main() {
-    let f = Foo; //~ ERROR
+    let f = Foo; //~ ERROR cannot find value `Foo` in this scope
+    //~^ NOTE not found in this scope
 }