Update library functions with stability attributes

This may not be strictly minimal, but all unstable functions also need a
`rustc_const_unstable` attribute.

library/proc_macro/src/lib.rs

@@ -20,6 +20,7 @@
 )]
 #![feature(nll)]
 #![feature(staged_api)]
+#![feature(const_fn)]
 #![feature(allow_internal_unstable)]
 #![feature(decl_macro)]
 #![feature(extern_types)]