Remove unused symbols and diagnostic items
As the title suggests, this removes unused symbols from `sym::` and `#[rustc_diagnostic_item]` annotations that weren't mentioned anywhere.
Originally I tried to use grep, to find symbols and item names that are never mentioned via `sym::name`, however this produced a lot of false positives (?), for example clippy matching on `Symbol::as_str` or macros "implicitly" adding `sym::`. I ended up fixing all these false positives (?) by hand, but tbh I'm not sure if it was worth it...
When encountering an unsatisfied trait bound, if there are no other
suggestions, mention all the types that *do* implement that trait:
```
error[E0277]: the trait bound `f32: Foo` is not satisfied
--> $DIR/impl_wf.rs:22:6
|
LL | impl Baz<f32> for f32 { }
| ^^^^^^^^ the trait `Foo` is not implemented for `f32`
|
= help: the following other types implement trait `Foo`:
Option<T>
i32
str
note: required by a bound in `Baz`
--> $DIR/impl_wf.rs:18:31
|
LL | trait Baz<U: ?Sized> where U: Foo { }
| ^^^ required by this bound in `Baz`
```
Mention implementers of traits in `ImplObligation`s.
Do not mention other `impl`s for closures, ranges and `?`.
