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/const_parameters/functions.rs

@@ -1,26 +1,29 @@
 // build-fail
-// compile-flags: -Zpolymorphize-errors
-#![feature(const_generics)]
+#![feature(const_generics, rustc_attrs)]
 //~^ WARN the feature `const_generics` is incomplete
 
 // This test checks that the polymorphization analysis correctly detects unused const
 // parameters in functions.
 
 // Function doesn't have any generic parameters to be unused.
+#[rustc_polymorphize_error]
 pub fn no_parameters() {}
 
 // Function has an unused generic parameter.
+#[rustc_polymorphize_error]
 pub fn unused<const T: usize>() {
-//~^ ERROR item has unused generic parameters
+    //~^ ERROR item has unused generic parameters
 }
 
 // Function uses generic parameter in value of a binding.
+#[rustc_polymorphize_error]
 pub fn used_binding<const T: usize>() -> usize {
     let x: usize = T;
     x
 }
 
 // Function uses generic parameter in substitutions to another function.
+#[rustc_polymorphize_error]
 pub fn used_substs<const T: usize>() {
     unused::<T>()
 }