MIR borrowck: spurious storagedead in constants

[arielb1]

For the issue-44373 run-pass test:

// Copyright 2017 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(bool);

struct Container(&'static [&'static Foo]);

static FOO: Foo = Foo(true);
static CONTAINER: Container = Container(&[&FOO]);

fn main() {}

The generated MIR for CONTAINER contains a StorageDead that should make the code illegal (and with MIR borrowck, makes it illegal):

static CONTAINER: Container = {
    let mut _0: Container;               // return place
    let mut _1: &[&Foo];
    let mut _2: &[&Foo; 1];
    let mut _3: &[&Foo; 1];
    let mut _4: [&Foo; 1];
    let mut _5: &Foo;
    let mut _6: &Foo;

    bb0: {                              
        StorageLive(_1);                 // bb0[0]: scope 0 at src/main.rs:16:41: 16:48
        StorageLive(_2);                 // bb0[1]: scope 0 at src/main.rs:16:41: 16:48
        StorageLive(_3);                 // bb0[2]: scope 0 at src/main.rs:16:41: 16:48
        StorageLive(_4);                 // bb0[3]: scope 0 at src/main.rs:16:42: 16:48
        StorageLive(_5);                 // bb0[4]: scope 0 at src/main.rs:16:43: 16:47
        StorageLive(_6);                 // bb0[5]: scope 0 at src/main.rs:16:43: 16:47
        _6 = &(FOO: Foo);                // bb0[6]: scope 0 at src/main.rs:16:43: 16:47
        _5 = _6;                         // bb0[7]: scope 0 at src/main.rs:16:43: 16:47
        _4 = [move _5];                  // bb0[8]: scope 0 at src/main.rs:16:42: 16:48
        _3 = &_4; // <- _4 is borrowed here and escapes:
        _2 = _3;                         // bb0[10]: scope 0 at src/main.rs:16:41: 16:48
        _1 = move _2 as &[&Foo] (Unsize); // bb0[11]: scope 0 at src/main.rs:16:41: 16:48
        _0 = Container::{{constructor}}(move _1,); // bb0[12]: scope 0 at src/main.rs:16:31: 16:49
        StorageDead(_3);                 // bb0[13]: scope 0 at src/main.rs:16:49: 16:49
        StorageDead(_4); // <- BUT, _4 is killed here
        StorageDead(_6);                 // bb0[15]: scope 0 at src/main.rs:16:49: 16:49
        return;                          // bb0[16]: scope 0 at src/main.rs:16:1: 16:50
    }

[arielb1]

cc @eddyb