Fix bad suggestions for code from proc_macro

[m-ou-se]

Fixes #85932

This disables these suggestions for spans from external macros, while keeping them for macros defined locally:

Before:

3 | #[hello]
  | ^^^^^^^^
  | |
  | expected `&mut i32`, found integer
  | help: consider mutably borrowing here: `&mut #[hello]`

After:

3 | #[hello]
  | ^^^^^^^^ expected `&mut i32`, found integer

Unchanged:

26 | macro_rules! bla { () => { x(123); } }
   |                              ^^^
   |                              |
   |                              expected `&mut i32`, found integer
   |                              help: consider mutably borrowing here: `&mut 123`
...
29 |     bla!();
   |     ------- in this macro invocation

[rust-highfive]

r? @jackh726

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[estebank]

r? @estebank
@bors r+

[bors]

📌 Commit e735f60 has been approved by estebank