librustc: Make references to functions not have static lifetime.

[pcwalton]

This breaks code like:

struct A<'a> {
    func: &'a fn() -> Option<int>
}

fn foo() -> Option<int> { ... }

fn create() -> A<'static> {
    A {
        func: &foo
    }
}

Change this code to not take functions by reference. For example:

struct A {
    func: extern "Rust" fn() -> Option<int>
}

fn foo() -> Option<int> { ... }

fn create() -> A {
    A {
        func: foo
    }
}

Closes #13595.

[breaking-change]

r? @huonw