Refactor EmitterWriter::emit_suggestion_default
Makes progress towards https://github.com/rust-lang/rust/issues/89979
Split into 2 commits:
* the first commit is purely a refactor and I verified that `./x.py test src/test/ui --stage 1` and `./x.py test src/test/rustdoc-ui --stage 1` continue to pass on this commit.
* ~~the second commit removes the empty trailing line from diff style suggestions.~~ - I discovered an issue with this so its just the refactor now.
r? diagnostics
626afa18b3