Make `ui/borrowck/borrowck-overloaded-call.rs` robust w.r.t. NLL.

src/test/ui/borrowck/borrowck-overloaded-call.nll.stderr

+error[E0502]: cannot borrow `s` as immutable because it is also borrowed as mutable
+  --> $DIR/borrowck-overloaded-call.rs:69:5
+   |
+LL |     let sp = &mut s;
+   |              ------ mutable borrow occurs here
+LL |     s(3);   //~ ERROR cannot borrow `s` as immutable because it is also borrowed as mutable
+   |     ^ immutable borrow occurs here
+LL |     use_mut(sp);
+   |             -- mutable borrow later used here
+
 error[E0596]: cannot borrow `s` as mutable, as it is not declared as mutable
   --> $DIR/borrowck-overloaded-call.rs:77:5
    |
@@ -17,7 +27,7 @@ LL |     s(" world".to_string());    //~ ERROR use of moved value: `s`
    |
    = note: move occurs because `s` has type `SFnOnce`, which does not implement the `Copy` trait
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-Some errors occurred: E0382, E0596.
+Some errors occurred: E0382, E0502, E0596.
 For more information about an error, try `rustc --explain E0382`.

src/test/ui/borrowck/borrowck-overloaded-call.rs

@@ -67,8 +67,8 @@ fn f() {
     };
     let sp = &mut s;
     s(3);   //~ ERROR cannot borrow `s` as immutable because it is also borrowed as mutable
+    use_mut(sp);
 }
-
 fn g() {
     let s = SFnMut {
         x: 1,
@@ -86,3 +86,5 @@ fn h() {
 }
 
 fn main() {}
+
+fn use_mut<T>(_: &mut T) { }

src/test/ui/borrowck/borrowck-overloaded-call.stderr

@@ -5,6 +5,7 @@ LL |     let sp = &mut s;
    |                   - mutable borrow occurs here
 LL |     s(3);   //~ ERROR cannot borrow `s` as immutable because it is also borrowed as mutable
    |     ^ immutable borrow occurs here
+LL |     use_mut(sp);
 LL | }
    | - mutable borrow ends here