Add a test for borrowck errors with multiple closure captures.

When converting check_loans to use ExprUseVisitor I encountered a few
issues where the wrong number of errors were being emitted for multiple
closure captures, but there is no existing test for this.

src/test/compile-fail/borrowck-multiple-captures.rs

@@ -0,0 +1,64 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::task;
+
+fn borrow<T>(_: &T) { }
+
+fn different_vars_after_borrows() {
+    let x1 = box 1;
+    let p1 = &x1;
+    let x2 = box 2;
+    let p2 = &x2;
+    task::spawn(proc() {
+        drop(x1); //~ ERROR cannot move `x1` into closure because it is borrowed
+        drop(x2); //~ ERROR cannot move `x2` into closure because it is borrowed
+    });
+    borrow(&*p1);
+    borrow(&*p2);
+}
+
+fn different_vars_after_moves() {
+    let x1 = box 1;
+    drop(x1);
+    let x2 = box 2;
+    drop(x2);
+    task::spawn(proc() {
+        drop(x1); //~ ERROR capture of moved value: `x1`
+        drop(x2); //~ ERROR capture of moved value: `x2`
+    });
+}
+
+fn same_var_after_borrow() {
+    let x = box 1;
+    let p = &x;
+    task::spawn(proc() {
+        drop(x); //~ ERROR cannot move `x` into closure because it is borrowed
+        drop(x); //~ ERROR use of moved value: `x`
+    });
+    borrow(&*p);
+}
+
+fn same_var_after_move() {
+    let x = box 1;
+    drop(x);
+    task::spawn(proc() {
+        drop(x); //~ ERROR capture of moved value: `x`
+        drop(x); //~ ERROR use of moved value: `x`
+    });
+}
+
+fn main() {
+    different_vars_after_borrows();
+    different_vars_after_moves();
+    same_var_after_borrow();
+    same_var_after_move();
+}
+