add an #[used] attribute

similar to GCC's __attribute((used))__. This attribute prevents LLVM from
optimizing away a non-exported symbol, within a compilation unit (object file),
when there are no references to it.

This is better explained with an example:

```
#[used]
static LIVE: i32 = 0;

static REFERENCED: i32 = 0;

static DEAD: i32 = 0;

fn internal() {}

pub fn exported() -> &'static i32 {
    &REFERENCED
}
```

Without optimizations, LLVM pretty much preserves all the static variables and
functions within the compilation unit.

```
$ rustc --crate-type=lib --emit=obj symbols.rs && nm -C symbols.o
0000000000000000 t drop::h1be0f8f27a2ba94a
0000000000000000 r symbols::REFERENCED::hb3bdfd46050bc84c
0000000000000000 r symbols::DEAD::hc2ea8f9bd06f380b
0000000000000000 r symbols::LIVE::h0970cf9889edb56e
0000000000000000 T symbols::exported::h6f096c2b1fc292b2
0000000000000000 t symbols::internal::h0ac1aadbc1e3a494
```

With optimizations, LLVM will drop dead code. Here `internal` is dropped because
it's not a exported function/symbol (i.e. not `pub`lic). `DEAD` is dropped for
the same reason. `REFERENCED` is preserved, even though it's not exported,
because it's referenced by the `exported` function. Finally, `LIVE` survives
because of the `#[used]` attribute even though it's not exported or referenced.

```
$ rustc --crate-type=lib -C opt-level=3 --emit=obj symbols.rs && nm -C symbols.o
0000000000000000 r symbols::REFERENCED::hb3bdfd46050bc84c
0000000000000000 r symbols::LIVE::h0970cf9889edb56e
0000000000000000 T symbols::exported::h6f096c2b1fc292b2
```

Note that the linker knows nothing about `#[used]` and will drop `LIVE`
because no other object references to it.

```
$ echo 'fn main() {}' >> symbols.rs
$ rustc symbols.rs && nm -C symbols | grep LIVE
```

At this time, `#[used]` only works on `static` variables.

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