wherein the status of empty and reason-only lint attributes is clarified

We avoid an ICE by checking for an empty meta-item list before we
index into the meta-items, and leave commentary about where we'd like
to issue unused-attributes lints in the future. Note that empty lint
attributes are already accepted by the stable compiler; generalizing
this to weird reason-only lint attributes seems like the
conservative/consilient generalization.

src/test/ui/lint/empty-lint-attributes.rs

@@ -0,0 +1,17 @@
+#![feature(lint_reasons)]
+
+// run-pass
+
+// Empty (and reason-only) lint attributes are legal—although we may want to
+// lint them in the future (Issue #55112).
+
+#![allow()]
+#![warn(reason = "observationalism")]
+
+#[forbid()]
+fn devoir() {}
+
+#[deny(reason = "ultion")]
+fn waldgrave() {}
+
+fn main() {}