Add ui test of explicit impl trait for type

Author: dtolnay

tests/ui/impl_trait_for_type.rs

@@ -0,0 +1,10 @@
+#[cxx::bridge]
+mod ffi {
+    struct S {
+        x: u8,
+    }
+
+    impl UniquePtrTarget for S {}
+}
+
+fn main() {}

tests/ui/impl_trait_for_type.stderr

@@ -0,0 +1,5 @@
+error: unexpected impl, expected something like `impl UniquePtr<T> {}`
+ --> $DIR/impl_trait_for_type.rs:7:10
+  |
+7 |     impl UniquePtrTarget for S {}
+  |          ^^^^^^^^^^^^^^^^^^^^^