syntax: Make `asm!` clobbers a proper vector.

Otherwise `--pretty expanded` diverges.
trans: Eliminated redundant allocations.
auto merge of #19411 : lifthrasiir/rust/asm-clobbers-expanded, r=alexcrichton

I.e. we should not prematurely build operand constraints at the expansion time. Otherwise `--pretty expanded` diverges:

```
$ cat t.rs
#![feature(asm)]

pub fn main() { unsafe { asm!("" : : : "hello", "world") }; }

$ rustc t.rs --pretty
#![feature(asm)]

pub fn main() { unsafe { asm!("" : : : "hello" , "world") }; }

$ rustc t.rs --pretty expanded
#![feature(asm)]
#![feature(phase)]
#![no_std]
#![feature(globs)]
#[phase(plugin, link)]
extern crate "std" as std;
#[prelude_import]
use std::prelude::*;

pub fn main() { unsafe { asm!("":  :  : "~{hello},~{world}") }; }
```

(The last code *does* compile, but won't do the expected thing.)