Handle GCC's write-only inline asm constraint in liveness, borrowck and trans.

I've implemented analysis support for the [GCC '=' write-only inline asm constraint modifier](http://gcc.gnu.org/onlinedocs/gcc/Modifiers.html). I had more changes, for '+' (read+write) as well, but it turns out LLVM doesn't support '+' at all.

I've removed the need for wrapping each output in ExprAddrOf, as that would require unwrapping almost everywhere and it was harder to reason about in borrowck than ExprAssign's LHS.

With this change, rustc will treat (in respect to validity of accessing a local) code like this:
```rust
let x: int;
unsafe {
    asm!("mov $1, $0" : "=r"(x) : "r"(5u));
}
```
as if it were this:
```rust
let x : int;
x = 5;
```

Previously, the local was required to be both mutable and initialized, and the write effect wasn't recorded.