Auto merge of #39987 - japaric:used, r=arielb1

#[used] attribute

(For an explanation of what this feature does, read the commit message)

I'd like to propose landing this as an experimental feature (experimental as in:
no clear stabilization path -- like `asm!`, `#[linkage]`) as it's low
maintenance (I think) and relevant to the "Usage in resource-constrained
environments" exploration area.

The main use case I see is running code before `main`. This could be used, for
instance, to cheaply initialize an allocator before `main` where the alternative
is to use `lazy_static` to initialize the allocator on its first use which it's
more expensive (atomics) and doesn't work on ARM Cortex-M0 microcontrollers (no
`AtomicUsize` on that platform)

Here's a `std` example of that:

``` rust

unsafe extern "C" fn before_main_1() {
    println!("Hello");
}

unsafe extern "C" fn before_main_2() {
    println!("World");
}

#[link_section = ".init_arary"]
#[used]
static INIT_ARRAY: [unsafe extern "C" fn(); 2] = [before_main_1, before_main_2];

fn main() {
    println!("Goodbye");
}
```

```
$ rustc -C lto -C opt-level=3 before_main.rs
$ ./before_main
Hello
World
Goodbye
```

In general, this pattern could be used to let *dependencies* run code before
`main` (which sounds like it could go very wrong in some cases). There are
probably other use cases; I hope that the people I have cc-ed can comment on
those.

Note that I'm personally unsure if the above pattern is something we want to
promote / allow and that's why I'm proposing this feature as experimental. If
this leads to more footguns than benefits then we can just axe the feature.

cc @nikomatsakis ^ I know you have some thoughts on having a process for
experimental features though I'm fine with writing an RFC before landing this.

- `dead_code` lint will have to be updated to special case `#[used]` symbols.

- Should we extend `#[used]` to work on non-generic functions?

cc rust-lang/rfcs#1002
cc rust-lang/rfcs#1459
cc @dpc @JinShil

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