Add ui test for reserved type names

tests/ui/reserved_name.rs

@@ -0,0 +1,16 @@
+#[cxx::bridge]
+mod ffi {
+    struct UniquePtr {
+        val: usize,
+    }
+
+    extern "C" {
+        type Box;
+    }
+
+    extern "Rust" {
+        type String;
+    }
+}
+
+fn main() {}

tests/ui/reserved_name.stderr

@@ -0,0 +1,17 @@
+error: reserved name
+ --> $DIR/reserved_name.rs:3:12
+  |
+3 |     struct UniquePtr {
+  |            ^^^^^^^^^
+
+error: reserved name
+ --> $DIR/reserved_name.rs:8:14
+  |
+8 |         type Box;
+  |              ^^^
+
+error: reserved name
+  --> $DIR/reserved_name.rs:12:14
+   |
+12 |         type String;
+   |              ^^^^^^