Incorrect spans for ABIs that do not match architecture

[nikomatsakis]

When an ABI does not include an option for the current architecture, we report an error but without any span information. We should not do that. See FIXMEs in the compiler.

See comment.

[huonw]

Triage: this seems to be fixed in that the error is reported with a span, except it points to the next token. E.g.

extern "foo" {}
type Foo = extern "foo" fn();
extern "foo" fn foo() {}

fn main() {}

8537.rs:1:13: 1:14 error: illegal ABI: expected one of [cdecl, stdcall, fastcall, aapcs, Rust, C, rust-intrinsic], found `foo`
8537.rs:1 extern "foo" {}
                       ^
8537.rs:2:24: 2:26 error: illegal ABI: expected one of [cdecl, stdcall, fastcall, aapcs, Rust, C, rust-intrinsic], found `foo`
8537.rs:2 type Foo = extern "foo" fn();
                                  ^~
8537.rs:3:13: 3:15 error: illegal ABI: expected one of [cdecl, stdcall, fastcall, aapcs, Rust, C, rust-intrinsic], found `foo`
8537.rs:3 extern "foo" fn foo() {}
                       ^~
error: aborting due to 3 previous errors
task '<unnamed>' failed at 'explicit failure', /home/huon/rust/src/libsyntax/diagnostic.rs:101
task '<unnamed>' failed at 'explicit failure', /home/huon/rust/src/librustc/rustc.rs:396

(repurposing this bug for the incorrect spans.)