Error spans for illegal extern "ABI"

```
pub extern "xxxxx" fn add(x: int, y: int) -> int {
    x + y
}
```

```
hello.rs:9:20: 9:22 error: illegal ABI: expected one of [cdecl, stdcall, fastcall, aapcs, win64, Rust, C, system, rust-intrinsic], found `xxxxx`
hello.rs:9 pub extern "xxxxx" fn add(x: int, y: int) -> int {
                              ^~
error: aborting due to previous error
```
