Add test for old ICE

The ICE from rust-lang#84044 no longer occurs.

src/test/ui/closures/issue-84044-drop-non-mut.rs

@@ -0,0 +1,6 @@
+// #84044: This used to ICE.
+
+fn main() {
+    let f = || {};
+    drop(&mut f); //~ ERROR cannot borrow `f` as mutable, as it is not declared as mutable
+}

src/test/ui/closures/issue-84044-drop-non-mut.stderr

@@ -0,0 +1,11 @@
+error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
+  --> $DIR/issue-84044-drop-non-mut.rs:5:10
+   |
+LL |     let f = || {};
+   |         - help: consider changing this to be mutable: `mut f`
+LL |     drop(&mut f);
+   |          ^^^^^^ cannot borrow as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.