Deduplicate code that finds the return type of a future

[tmandry]

I've noticed a lot of code being added to different parts of the compiler that all does the same thing.. gives you the return type of an async fn. This issue is to keep track of all those places and eventually deduplicate them.

[guswynn]

Oh I noticed this as well on #78297, forgot to make an issue for it!

[tmandry]

Another place that was added: here in #76468