fix: Match rustc's annotation overlap

[Muscraft]

While working on rustc, I saw that annotate-snippets and rustc had different outputs for unsafe_extern_suggestion. I spent some time digging into this, and it turns out we did not match how rustc handled overlaps. Some of the ways I attempted to fix this divergence caused alloc_error_handler_bad_signature_2 and str_escape to regress, so I figured I would add them to help catch any future changes.