librustc: Record unique immutable borrows in the restrictions table.

This fixes borrow checking for closures. Code like this will break:

    struct Foo {
        x: int,
    }

    pub fn main() {
        let mut this = &mut Foo {
            x: 1,
        };
        let r = || {
            let p = &this.x;
            &mut this.x;
        };
        r()
    }

Change this code to not take multiple mutable references to the same value. For
example:

    struct Foo {
        x: int,
    }

    pub fn main() {
        let mut this = &mut Foo {
            x: 1,
        };
        let r = || {
            &mut this.x;
        };
        r()
    }

Closes #16361.

[breaking-change]

src/test/compile-fail/borrowck-closures-unique-imm.rs

@@ -0,0 +1,25 @@
+// 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.
+
+struct Foo {
+    x: int,
+}
+
+pub fn main() {
+    let mut this = &mut Foo {
+        x: 1,
+    };
+    let r = || {
+        let p = &this.x;
+        &mut this.x; //~ ERROR cannot borrow
+    };
+    r()
+}
+