Tell tidy about compiler_builtins_lib feature

After the work in rust-lang#42899, it no longer auto-discovers it.

src/tools/tidy/src/features.rs

@@ -219,6 +219,18 @@ pub fn collect_lang_features(base_src_path: &Path) -> Features {
 
 pub fn collect_lib_features(base_src_path: &Path) -> Features {
     let mut lib_features = Features::new();
+
+    // This library feature is defined in the `compiler_builtins` crate, which
+    // has been moved out-of-tree. Now it can no longer be auto-discovered by
+    // `tidy`, because we need to filter out its (submodule) directory. Manually
+    // add it to the set of known library features so we can still generate docs.
+    lib_features.insert("compiler_builtins_lib".to_owned(), Feature {
+        level: Status::Unstable,
+        since: "".to_owned(),
+        has_gate_test: false,
+        tracking_issue: None,
+    });
+
     map_lib_features(base_src_path,
                      &mut |res, _, _| {
         match res {