syntax: Fix span on illegal ABI errors

[lucy]

Fixes #8537
Fixes #14499 (duplicate of #8537)

Old:

test.rs:2   pub extern "xxxxx" fn add(x: int, y: int) -> int {
                               ^~

New:

test.rs:2   pub extern "xxxxx" fn add(x: int, y: int) -> int {
                       ^~~~~~~

[alexcrichton]

Can you add a test for this as well? A test may look like

// src/test/compile-fail/foo.rs
pub extern
  "invalid-abi" //~ ERROR: ...
fn foo() {}

fn main() {}

[lucy]

@alexcrichton done