Update test for issue 17780 since diagnostic message have changed

The test was also renamed to be more descriptive.

Author: bkoropoff

src/test/compile-fail/issue-17780.rs renamed to src/test/compile-fail/borrow-immutable-upvar-mutation.rs

@@ -10,41 +10,32 @@
 
 #![feature(unboxed_closures, overloaded_calls)]
 
+// Tests that we can't assign to or mutably borrow upvars from `Fn`
+// closures (issue #17780)
+
 fn set(x: &mut uint) { *x = 5; }
 
 fn main() {
     // By-ref captures
     {
         let mut x = 0u;
-        let _f = |&:| x = 42;
-        //~^ ERROR cannot assign to data in a free
-        // variable from an immutable unboxed closure
+        let _f = |&:| x = 42; //~ ERROR cannot assign
 
         let mut y = 0u;
-        let _g = |&:| set(&mut y);
-        //~^ ERROR cannot borrow data mutably in a free
-        // variable from an immutable unboxed closure
+        let _g = |&:| set(&mut y); //~ ERROR cannot borrow
 
         let mut z = 0u;
-        let _h = |&mut:| { set(&mut z); |&:| z = 42; };
-        //~^ ERROR cannot assign to data in a
-        // free variable from an immutable unboxed closure
+        let _h = |&mut:| { set(&mut z); |&:| z = 42; }; //~ ERROR cannot assign
     }
     // By-value captures
     {
         let mut x = 0u;
-        let _f = move |&:| x = 42;
-        //~^ ERROR cannot assign to data in a free
-        // variable from an immutable unboxed closure
+        let _f = move |&:| x = 42; //~ ERROR cannot assign
 
         let mut y = 0u;
-        let _g = move |&:| set(&mut y);
-        //~^ ERROR cannot borrow data mutably in a free
-        // variable from an immutable unboxed closure
+        let _g = move |&:| set(&mut y); //~ ERROR cannot borrow
 
         let mut z = 0u;
-        let _h = move |&mut:| { set(&mut z); move |&:| z = 42; };
-        //~^ ERROR cannot assign to data in a free
-        // variable from an immutable unboxed closure
+        let _h = move |&mut:| { set(&mut z); move |&:| z = 42; }; //~ ERROR cannot assign
     }
 }