Make #[rustc_private] override dependency visibility

Really this is always-visible override only needs to happen when the
crate is a dependency of itself. However, this is a very internal
feature, so it doesn't seem worth doing any additional filtering here.

compiler/rustc_middle/src/ty/util.rs

@@ -876,6 +876,11 @@ impl<'tcx> TyCtxt<'tcx> {
     /// [public]: TyCtxt::is_private_dep
     /// [direct]: rustc_session::cstore::ExternCrate::is_direct
     pub fn is_user_visible_dep(self, key: CrateNum) -> bool {
+        // `#![rustc_private]` overrides defaults to make private dependencies usable.
+        if self.features().enabled(sym::rustc_private) {
+            return true;
+        }
+
         // | Private | Direct | Visible |                    |
         // |---------|--------|---------|--------------------|
         // | Yes     | Yes    | Yes     | !true || true   |