Fill in ignore reasons in all #[ignore] attributes

Author: dtolnay

tests/compiletest.rs

@@ -1,7 +1,7 @@
 #[allow(unused_attributes)]
-#[rustversion::attr(not(nightly), ignore)]
-#[cfg_attr(skip_ui_tests, ignore)]
-#[cfg_attr(miri, ignore)]
+#[rustversion::attr(not(nightly), ignore = "requires nightly")]
+#[cfg_attr(skip_ui_tests, ignore = "disabled by `--cfg=skip_ui_tests`")]
+#[cfg_attr(miri, ignore = "incompatible with miri")]
 #[test]
 fn ui() {
     let t = trybuild::TestCases::new();