Record all declared features.

Currently `rust_20XX_preview` features aren't recorded as declared even
when they are explicit declared. Similarly, redundant edition-dependent
features (e.g. `test_2018_feature`) aren't recorded as declared.

This commit marks them as recorded. There is no detectable functional
change, but it makes things more consistent.

compiler/rustc_expand/src/config.rs

@@ -124,6 +124,8 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
                         edition,
                     });
                 }
+                features.declared_lang_features.push((name, mi.span(), None));
+                features.declared_features.insert(name);
                 continue;
             }
 
@@ -137,6 +139,8 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
                     feature: name,
                     edition: features_edition,
                 });
+                features.declared_lang_features.push((name, mi.span(), None));
+                features.declared_features.insert(name);
                 continue;
             }