Rollup merge of #104006 - flip1995:lang-items-clippy, r=oli-obk

Rollup merge of #104006 - flip1995:lang-items-clippy, r=oli-obk

Add variant_name function to `LangItem`

Clippy has an internal lint that checks for the usage of hardcoded def paths and suggests to replace them with a lang or diagnostic item, if possible. This was implemented with a hack, by getting all the variants of the `LangItem` enum and then index into it with the position of the `LangItem` in the `items` list. This is no longer possible, because the `items` list can't be accessed anymore.

Follow up to #103603

cc `@camsteffen`
r? `@oli-obk`

This is blocking the sync between Clippy and Rust. I'm not sure if this is the best solution here, or if I should add a method `items()` to `LanguageItems` and keep the code in Clippy unchanged.