Point at the `Fn()` or `FnMut()` bound that coerced a closure, which caused a move error

When encountering a move error involving a closure because the captured value isn't `Copy`, and the obligation comes from a bound on a type parameter that requires `Fn` or `FnMut`, we point at it and explain that an `FnOnce` wouldn't cause the move error.

```
error[E0507]: cannot move out of `foo`, a captured variable in an `Fn` closure
  --> f111.rs:15:25
   |
14 | fn do_stuff(foo: Option<Foo>) {
   |             ---  ----------- move occurs because `foo` has type `Option<Foo>`, which does not implement the `Copy` trait
   |             |
   |             captured outer variable
15 |     require_fn_trait(|| async {
   |                      -- ^^^^^ `foo` is moved here
   |                      |
   |                      captured by this `Fn` closure
16 |         if foo.map_or(false, |f| f.foo()) {
   |            --- variable moved due to use in coroutine
   |
help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
  --> f111.rs:12:53
   |
12 | fn require_fn_trait<F: Future<Output = ()>>(_: impl Fn() -> F) {}
   |                                                     ^^^^^^^^^
help: consider cloning the value if the performance cost is acceptable
   |
16 |         if foo.clone().map_or(false, |f| f.foo()) {
   |               ++++++++
```

1 files changed, 4 insertions, 3 deletions

diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index 5a5563c7bb2..98be37fd84b 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -847,17 +847,18 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
         self
     }
 
+    with_fn! { with_span_help,
     /// Prints the span with some help above it.
     /// This is like [`Diag::help()`], but it gets its own span.
     #[rustc_lint_diagnostics]
-    pub fn span_help<S: Into<MultiSpan>>(
+    pub fn span_help(
         &mut self,
-        sp: S,
+        sp: impl Into<MultiSpan>,
         msg: impl Into<SubdiagMessage>,
     ) -> &mut Self {
         self.sub(Level::Help, msg, sp.into());
         self
-    }
+    } }
 
     /// Disallow attaching suggestions to this diagnostic.
     /// Any suggestions attached e.g. with the `span_suggestion_*` methods