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 `?`.

Author: estebank

core/src/ops/function.rs

@@ -53,6 +53,7 @@
 /// ```
 #[lang = "fn"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "Fn"]
 #[rustc_paren_sugar]
 #[rustc_on_unimplemented(
     on(
@@ -133,6 +134,7 @@ pub trait Fn<Args>: FnMut<Args> {
 /// ```
 #[lang = "fn_mut"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "FnMut"]
 #[rustc_paren_sugar]
 #[rustc_on_unimplemented(
     on(
@@ -205,6 +207,7 @@ pub trait FnMut<Args>: FnOnce<Args> {
 /// ```
 #[lang = "fn_once"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "FnOnce"]
 #[rustc_paren_sugar]
 #[rustc_on_unimplemented(
     on(

core/src/slice/index.rs

@@ -140,6 +140,7 @@ mod private_slice_index {
 /// Implementations of this trait have to promise that if the argument
 /// to `get_(mut_)unchecked` is a safe reference, then so is the result.
 #[stable(feature = "slice_get_slice", since = "1.28.0")]
+#[rustc_diagnostic_item = "SliceIndex"]
 #[rustc_on_unimplemented(
     on(T = "str", label = "string indices are ranges of `usize`",),
     on(