mir: use attribute over -Z polymorphize-errors

This commit replaces the `-Z polymorphize-errors` debugging flag with a
`#[rustc_polymorphize_error]` attribute for use on functions.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/polymorphization/predicates.rs

@@ -1,13 +1,15 @@
 // build-fail
-// compile-flags: -Zpolymorphize-errors
+#![feature(rustc_attrs)]
 
 // This test checks that `T` is considered used in `foo`, because it is used in a predicate for
 // `I`, which is used.
 
+#[rustc_polymorphize_error]
 fn bar<I>() {
-//~^ ERROR item has unused generic parameters
+    //~^ ERROR item has unused generic parameters
 }
 
+#[rustc_polymorphize_error]
 fn foo<I, T>(_: I)
 where
     I: Iterator<Item = T>,