Rollup merge of #139880 - compiler-errors:rpitit-nameless, r=nnethercote

Don't compute name of associated item if it's an RPITIT

Use `Option::then` in favor of `Option::then_some` to not compute `AssocItem::name` if it fails the condition. Alternatively, I'd be open to changing this just to an `if`.

Fixes https://github.com/rust-lang/rust/issues/139873

r? ```@nnethercote```