Improve span of errors on implicitly generated shim impls

[dtolnay]

Repro:

pub struct MyBuilder<'a> {
    _s: &'a str,
}

type OptBuilder<'a> = Option<MyBuilder<'a>>;

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        type OptBuilder<'a>;
    }

    struct MyBuilder<'a> {
        rs: Box<OptBuilder<'a>>,
    }
}

Diagnostic before:

Arrows everywhere, completely confused spans.

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
  --> $DIR/nonlocal_rust_type.rs:7:1
   |
7  |   #[cxx::bridge]
   |   ^^^^^^^^^^^^^^ impl doesn't use only types from inside the current crate
...
10 |           type OptBuilder<'a>;
   |  ___________________________-
11 | |     }
12 | |
13 | |     struct MyBuilder<'a> {
14 | |         rs: Box<OptBuilder<'a>>,
   | |__________________________- `Option` is not defined in the current crate
   |
   = note: define and implement a trait or new type instead
   = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

Diagnostic after

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
  --> $DIR/nonlocal_rust_type.rs:14:13
   |
14 |         rs: Box<OptBuilder<'a>>,
   |             ^^^^---------------
   |             |   |
   |             |   `Option` is not defined in the current crate
   |             impl doesn't use only types from inside the current crate
   |
   = note: define and implement a trait or new type instead