Add optional span to `require_lang_item` for diagnostics with more context

[`fn require_lang_item(LangItem)`](https://github.com/rust-lang/rust/blob/615c46086a994f088c9ed569fc36df229ae115b6/src/librustc/middle/lang_items.rs#L384-L388) should also take an optional `Span` to [avoid output without context](https://github.com/rust-lang/rust/pull/63917/files#diff-94321333c9f60a3538b4f566849c232e).