Mention implementers of unsatisfied trait

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 `?`.

compiler/rustc_errors/src/diagnostic.rs

@@ -378,6 +378,12 @@ impl Diagnostic {
         self
     }
 
+    /// Add a help message attached to this diagnostic with a customizable highlighted message.
+    pub fn highlighted_help(&mut self, msg: Vec<(String, Style)>) -> &mut Self {
+        self.sub_with_highlights(Level::Help, msg, MultiSpan::new(), None);
+        self
+    }
+
     /// Prints the span with some help above it.
     /// This is like [`Diagnostic::help()`], but it gets its own span.
     pub fn span_help<S: Into<MultiSpan>>(&mut self, sp: S, msg: &str) -> &mut Self {