Add a test for casts of univariant C-like enums

Author: nox

src/test/run-pass/issue-23304-2.rs

@@ -10,8 +10,13 @@
 
 #![allow(dead_code)]
 
-enum X { A = 0 as isize }
+enum X { A = 42 as isize }
 
 enum Y { A = X::A as isize }
 
-fn main() { }
+fn main() {
+    let x = X::A;
+    let x = x as isize;
+    assert_eq!(x, 42);
+    assert_eq!(Y::A as isize, 42);
+}