Suggest correct place to add `self` parameter when inside closure
It would incorrectly suggest adding it as a parameter to the closure instead of the containing function.
[For example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=1936bcd1e5f981573386e0cee985c3c0):
```
help: add a `self` receiver parameter to make the associated `fn` a method
|
5 | let _ = || self&self;
| ^^^^^
```
`DiagnosticMetadata.current_function` is only used for these messages so tweaking its behavior should be ok.
57e38dd4fb