Check const-propagation of borrows of unsized places

src/test/ui/consts/const-prop-ice3.rs

@@ -0,0 +1,7 @@
+// run-pass (ensure that const-prop is run)
+
+struct A<T: ?Sized>(T);
+
+fn main() {
+    let _x = &(&A([2, 3]) as &A<[i32]>).0 as *const [i32] as *const i32;
+}