Add test for rust-lang#124651

Author: estebank

tests/ui/diagnostic_namespace/auxiliary/bad_on_unimplemented.rs

@@ -0,0 +1,2 @@
+#[diagnostic::on_unimplemented(aa = "broken")]
+pub trait Test {}

tests/ui/diagnostic_namespace/on_unimplemented_ice.rs

@@ -0,0 +1,17 @@
+//@ edition:2021
+//@ compile-flags:--test
+//@ aux-build:bad_on_unimplemented.rs
+
+// Do not ICE when encountering a malformed `#[diagnostic::on_unimplemented]` annotation in a
+// dependency when incorrectly used (#124651).
+
+extern crate bad_on_unimplemented;
+
+use bad_on_unimplemented::Test;
+
+fn breakage<T: Test>(_: T) {}
+
+#[test]
+fn test() {
+    breakage(1); //~ ERROR E0277
+}