[nll] librustc_incremental: enable feature(nll) for bootstrap

Author: memoryruins

src/librustc_incremental/lib.rs

@@ -14,6 +14,7 @@
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/")]
 
+#![cfg_attr(not(stage0), feature(nll))]
 #![feature(specialization)]
 
 #![recursion_limit="256"]