10798: ide: show go to for function hover return type r=Veykril a=jhgg
I've found myself wanting this... adds to the hover quick go-to for a function's return type:
![image](https://user-images.githubusercontent.com/5489149/142375722-4a385216-494b-45a4-be1c-59664213b8d6.png)
This is particularly useful when you are hovering over a function in a long chain, like:
```rust
foo.bar().b$0az().some_trait_fn();
```
where `baz`'s return type is not immediately obvious, but the chain is not long enough to trigger chain inlay hints...
i guess I could just select `foo.bar().baz()` too to get the types too...
Co-authored-by: Jake Heinz <jh@discordapp.com>