Auto merge of #29014 - petrochenkov:stability, r=brson

Stricter checking of stability attributes + enforcement of their invariants at compile time
(+ removed dead file librustc_front/attr.rs)
I intended to enforce use of `reason` for unstable items as well (it normally presents for new items), but it turned out too intrusive, many older unstable items don't have `reason`s.

r? @aturon 
I'm studying how stability works and do some refactoring along the way, so it's probably not the last PR.

src/libsyntax/ext/base.rs

@@ -587,7 +587,7 @@ impl<'a> ExtCtxt<'a> {
         }
     }
 
-    #[unstable(feature = "rustc_private")]
+    #[unstable(feature = "rustc_private", issue = "0")]
     #[deprecated(since = "1.0.0",
                  reason = "Replaced with `expander().fold_expr()`")]
     pub fn expand_expr(&mut self, e: P<ast::Expr>) -> P<ast::Expr> {