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]

r? @nikomatsakis