add test for rust issue 94371

Author: RalfJung

tests/run-pass/issue-94371.rs

@@ -0,0 +1,13 @@
+#[repr(C)]
+struct Demo(u64, bool, u64, u32, u64, u64, u64);
+
+fn test() -> (Demo, Demo) {
+    let mut x = Demo(1, true, 3, 4, 5, 6, 7);
+    let mut y = Demo(10, false, 12, 13, 14, 15, 16);
+    std::mem::swap(&mut x, &mut y);
+    (x, y)
+}
+
+fn main() {
+    drop(test());
+}