test that creating a 2nd mutable ref from a NonNull invalidates the first

tests/compile-fail/stacked_borrows/mut_exclusive_violation2.rs

@@ -0,0 +1,10 @@
+use std::ptr::NonNull;
+
+fn main() { unsafe {
+    let x = &mut 0;
+    let mut ptr1 = NonNull::from(x);
+    let mut ptr2 = ptr1.clone();
+    let raw1 = ptr1.as_mut();
+    let _raw2 = ptr2.as_mut();
+    let _val = *raw1; //~ ERROR borrow stack
+} }