librustc: Record unique immutable borrows in the restrictions table.

[pcwalton]

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]

r? @nikomatsakis