librustc: Put #[unsafe_destructor] behind a feature gate.

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]

src/libsyntax/lib.rs

@@ -27,9 +27,10 @@ This API is completely unstable and subject to change.
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",
        html_root_url = "http://doc.rust-lang.org/")]
 
-#![feature(macro_rules, globs, managed_boxes, default_type_params, phase,
-           quote)]
+#![feature(macro_rules, globs, managed_boxes, default_type_params, phase)]
+#![feature(quote, unsafe_destructor)]
 #![allow(deprecated)]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 extern crate serialize;
 extern crate term;