expand: Change how #![cfg(FALSE)] behaves on crate root

Previously it removed all other attributes from the crate root.
Now it removes only attributes below itself.

So it becomes possible to configure some global crate properties even for fully unconfigured crates.

compiler/rustc_expand/src/config.rs

@@ -197,9 +197,11 @@ pub fn pre_configure_attrs(sess: &Session, attrs: &[Attribute]) -> ast::AttrVec
         config_tokens: false,
         lint_node_id: ast::CRATE_NODE_ID,
     };
-    let attrs: ast::AttrVec =
-        attrs.iter().flat_map(|attr| strip_unconfigured.process_cfg_attr(attr)).collect();
-    if strip_unconfigured.in_cfg(&attrs) { attrs } else { ast::AttrVec::new() }
+    attrs
+        .iter()
+        .flat_map(|attr| strip_unconfigured.process_cfg_attr(attr))
+        .take_while(|attr| !is_cfg(attr) || strip_unconfigured.cfg_true(attr).0)
+        .collect()
 }
 
 #[macro_export]