add nll compare mode stderr file

Author: guswynn

src/test/ui/issues/issue-76547.nll.stderr

@@ -0,0 +1,20 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-76547.rs:19:14
+   |
+LL | async fn fut(bufs: &mut [&mut [u8]]) {
+   |              ^^^^  -     - let's call the lifetime of this reference `'2`
+   |              |     |
+   |              |     let's call the lifetime of this reference `'1`
+   |              assignment requires that `'1` must outlive `'2`
+
+error: lifetime may not live long enough
+  --> $DIR/issue-76547.rs:33:15
+   |
+LL | async fn fut2(bufs: &mut [&mut [u8]]) -> i32 {
+   |               ^^^^  -     - let's call the lifetime of this reference `'2`
+   |               |     |
+   |               |     let's call the lifetime of this reference `'1`
+   |               assignment requires that `'1` must outlive `'2`
+
+error: aborting due to 2 previous errors
+