When finding item gated behind a `cfg` flat, point at it

Previously we would only mention that the item was gated out, and opportunisitically mention the feature flag name when possible. We now point to the place where the item was gated, which can be behind layers of macro indirection, or in different modules.

```
error[E0433]: failed to resolve: could not find `doesnt_exist` in `inner`
  --> $DIR/diagnostics-cross-crate.rs:18:23
   |
LL |     cfged_out::inner::doesnt_exist::hello();
   |                       ^^^^^^^^^^^^ could not find `doesnt_exist` in `inner`
   |
note: found an item that was configured out
  --> $DIR/auxiliary/cfged_out.rs:6:13
   |
LL |     pub mod doesnt_exist {
   |             ^^^^^^^^^^^^
note: the item is gated here
  --> $DIR/auxiliary/cfged_out.rs:5:5
   |
LL |     #[cfg(FALSE)]
   |     ^^^^^^^^^^^^^
```

1 files changed, 7 insertions, 1 deletions

diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index 09221041031..3ce500866bd 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -2530,7 +2530,13 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 && let NestedMetaItem::MetaItem(meta_item) = &nested[0]
                 && let MetaItemKind::NameValue(feature_name) = &meta_item.kind
             {
-                let note = errors::ItemWasBehindFeature { feature: feature_name.symbol };
+                let note = errors::ItemWasBehindFeature {
+                    feature: feature_name.symbol,
+                    span: meta_item.span,
+                };
+                err.subdiagnostic(note);
+            } else {
+                let note = errors::ItemWasCfgOut { span: cfg.span };
                 err.subdiagnostic(note);
             }
         }