Rollup merge of #117173 - oli-obk:gen_fn_split2, r=compiler-errors

Make `Iterator` a lang item

r? `@compiler-errors`

pulled out of https://github.com/rust-lang/rust/pull/116447

We're doing this change on its own, because iterator was the one diagnostic item that was load bearing on us correctly emitting errors about `diagnostic_item` mis-uses. It was used in some diagnostics as an early abort, before the actual checks of the diagnostic, so effectively the compiler was *unconditionally* checking for the iterator diagnostic item, even if it didn't emit any diagnostics. Changing those uses to use the lang item, caused us not to invoke the `all_diagnostic_items` query anymore, which then caused us to miss some issues around diagnostic items until they were actually used.

The reason we keep the diagnostic item around is that clippy uses it a lot and having `Iterator` be a lang item and a diagnostic item at the same time doesn't cost us anything, but makes clippy's internal code simpler

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