Update E0379 to new format rust-lang#35338

Author: kc1212

src/librustc_typeck/check/mod.rs

@@ -847,7 +847,9 @@ fn check_trait_fn_not_const<'a,'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
             // good
         }
         hir::Constness::Const => {
-            span_err!(ccx.tcx.sess, span, E0379, "trait fns cannot be declared const");
+            struct_span_err!(ccx.tcx.sess, span, E0379, "trait fns cannot be declared const")
+                .span_label(span, &format!("trait fns cannot be const"))
+                .emit()
         }
     }
 }

src/test/compile-fail/const-fn-mismatch.rs

@@ -20,7 +20,9 @@ trait Foo {
 }
 
 impl Foo for u32 {
-    const fn f() -> u32 { 22 } //~ ERROR E0379
+    const fn f() -> u32 { 22 }
+    //~^ ERROR E0379
+    //~| NOTE trait fns cannot be const
 }
 
 fn main() { }