Rollup merge of #120143 - compiler-errors:consolidate-instance-resolv…

…e-for-coroutines, r=oli-obk

Consolidate logic around resolving built-in coroutine trait impls

Deduplicates a lot of code. Requires defining a new lang item for `Coroutine::resume` for consistency, but it seems not harmful at worst, and potentially later useful at best.

r? oli-obk