Auto merge of #72559 - Aaron1011:feature/assoc-lang-items, r=oli-obk

Auto merge of #72559 - Aaron1011:feature/assoc-lang-items, r=oli-obk

Implement associated lang items

Fixes #70718

This commit allows making associated items (e.g. associated functions
and types) into lang items via the `#[lang]` attribute. This allows such
items to be accessed directly, rather than by iterating over the parent
item's associated items.

I've added `FnOnce::Output` as a lang item, and updated one old usage to
use the new lang item. The remaining uses can be updated separately.