annotate-snippet emitter: Deal with multispans in macros correctly

[phansch]

Part of #59346

To properly handle macro spans in diagnostics output, we have to decouple some emitter.rs code first.

Relevant FIXME:

rust/src/librustc_errors/annotate_snippet_emitter_writer.rs

Line 38 in 57a3300

// FIXME(#59346): Add `fix_multispans_in_std_macros` function from emitter.rs

emitter.rs equivalent:

rust/src/librustc_errors/emitter.rs

Line 792 in 0e4a56b

fn fix_multispans_in_std_macros(&mut self,

• We first have to decouple the fix_multispans_in_std_macros
  method from EmitterWriter so that the code can be re-used in
  AnnotateSnippetEmitterWriter without duplicating it.

[phansch]

@rustbot modify labels: A-diagnostics E-help-wanted