Add ui test of invalid nonempty impl block

Author: dtolnay

tests/ui/nonempty_impl_block.rs

@@ -0,0 +1,12 @@
+#[cxx::bridge]
+mod ffi {
+    struct S {
+        x: u8,
+    }
+
+    impl UniquePtr<S> {
+        fn new() -> Self;
+    }
+}
+
+fn main() {}

tests/ui/nonempty_impl_block.stderr

@@ -0,0 +1,8 @@
+error: expected an empty impl block
+ --> $DIR/nonempty_impl_block.rs:7:23
+  |
+7 |       impl UniquePtr<S> {
+  |  _______________________^
+8 | |         fn new() -> Self;
+9 | |     }
+  | |_____^