Add regression test for doc cfg applied on public items inside private items

tests/rustdoc/doc-auto-cfg-public-in-private.rs

@@ -0,0 +1,16 @@
+// This test ensures that even though private items are removed from generated docs,
+// their `cfg`s will still impact their child items.
+
+#![feature(doc_cfg)]
+#![crate_name = "foo"]
+
+pub struct X;
+
+#[cfg(not(feature = "blob"))]
+fn foo() {
+    impl X {
+        //@ has 'foo/struct.X.html'
+        //@ has - '//*[@class="stab portability"]' 'Available on non-crate feature blob only.'
+        pub fn bar() {}
+    }
+}