Auto merge of #41745 - oli-obk:diagnostics, r=jonathandturner

Remove need for &format!(...) or &&"" dances in `span_label` calls

These were always a thorn in my eye. Note that this will monomorphize to two impls, one for `String` and one for `&str`. But I think that cost is worth the ergonomics at the call sites that can be seen throughout this PR.