error code examples: replace some ignore with compile_fail

src/librustc_error_codes/error_codes/E0454.md

@@ -1,6 +1,6 @@
 A link name was given with an empty name. Erroneous code example:
 
-```ignore (cannot-test-this-because-rustdoc-stops-compile-fail-before-codegen)
+```compile_fail,E0454
 #[link(name = "")] extern {}
 // error: `#[link(name = "")]` given with empty name
 ```

src/librustc_error_codes/error_codes/E0458.md

@@ -1,6 +1,6 @@
 An unknown "kind" was specified for a link attribute. Erroneous code example:
 
-```ignore (cannot-test-this-because-rustdoc-stops-compile-fail-before-codegen)
+```compile_fail,E0458
 #[link(kind = "wonderful_unicorn")] extern {}
 // error: unknown kind: `wonderful_unicorn`
 ```

src/librustc_error_codes/error_codes/E0459.md

@@ -1,6 +1,6 @@
 A link was used without a name parameter. Erroneous code example:
 
-```ignore (cannot-test-this-because-rustdoc-stops-compile-fail-before-codegen)
+```compile_fail,E0459
 #[link(kind = "dylib")] extern {}
 // error: `#[link(...)]` specified without `name = "foo"`
 ```