Adding compile fail test for staged_api feature

Issue #39059
r? @est31

src/test/compile-fail/feature-gate-staged_api.rs

+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![stable(feature = "a", since = "b")]
+//~^ ERROR stability attributes may not be used outside of the standard library
+mod inner_private_module {
+    // UnnameableTypeAlias isn't marked as reachable, so no stability annotation is required here
+    pub type UnnameableTypeAlias = u8;
+}
+
+#[stable(feature = "a", since = "b")]
+//~^ ERROR stability attributes may not be used outside of the standard library
+pub fn f() -> inner_private_module::UnnameableTypeAlias {
+    0
+}
+
+fn main() {}

src/tools/tidy/src/features.rs

@@ -167,7 +167,7 @@ pub fn check(path: &Path, bad: &mut bool) {
     // FIXME get this whitelist empty.
     let whitelist = vec![
         "abi_ptx", "simd", "static_recursion",
-        "cfg_target_has_atomic", "staged_api",
+        "cfg_target_has_atomic",
         "unboxed_closures", "stmt_expr_attributes",
         "cfg_target_thread_local", "unwind_attributes",
         "inclusive_range_syntax"