rustdoc-json: added a test for rust-lang#107278

Author: its-the-shrimp

tests/rustdoc-json/impls/pub_for_hidden_private.rs

@@ -0,0 +1,10 @@
+//@ compile-flags: --document-private-items --document-hidden-items
+
+pub trait TheTrait {}
+
+#[doc(hidden)]
+struct Value {}
+
+//@ has '$.index[*][?(@.docs=="THE IMPL")]'
+/// THE IMPL
+impl TheTrait for Value {}