Rollup merge of #87322 - chazkiker2:fix/suggestion-ref-sync-send, r=estebank

fix: clarify suggestion that `&T` must refer to `T: Sync` for `&T: Send`

### Description

- [x] fix #86507
- [x] add UI test for relevant code from issue
- [x] change `rustc_trait_selection/src/traits/error_reporting/suggestions.rs` to include a more clear suggestion when `&T` fails to satisfy `Send` bounds due to the fact that `T` fails to implement `Sync`
- [x] update UI test in Clippy: `src/tools/tests/ui/future_not_send.stderr`

src/test/ui/async-await/issue-86507.rs

@@ -0,0 +1,25 @@
+// edition:2018
+
+use ::core::pin::Pin;
+use ::core::future::Future;
+use ::core::marker::Send;
+
+trait Foo {
+    fn bar<'me, 'async_trait, T: Send>(x: &'me T)
+        -> Pin<Box<dyn Future<Output = ()> + Send + 'async_trait>>
+        where 'me: 'async_trait;
+}
+
+impl Foo for () {
+    fn bar<'me, 'async_trait, T: Send>(x: &'me T)
+        -> Pin<Box<dyn Future<Output = ()> + Send + 'async_trait>>
+        where 'me:'async_trait {
+            Box::pin( //~ ERROR future cannot be sent between threads safely
+                async move {
+                    let x = x;
+                }
+            )
+         }
+}
+
+fn main() { }

src/test/ui/async-await/issue-86507.stderr

@@ -0,0 +1,23 @@
+error: future cannot be sent between threads safely
+  --> $DIR/issue-86507.rs:17:13
+   |
+LL | /             Box::pin(
+LL | |                 async move {
+LL | |                     let x = x;
+LL | |                 }
+LL | |             )
+   | |_____________^ future created by async block is not `Send`
+   |
+note: captured value is not `Send` because `&` references cannot be sent unless their referent is `Sync`
+  --> $DIR/issue-86507.rs:19:29
+   |
+LL |                     let x = x;
+   |                             ^ has type `&T` which is not `Send`, because `T` is not `Sync`
+   = note: required for the cast to the object type `dyn Future<Output = ()> + Send`
+help: consider further restricting type parameter `T`
+   |
+LL |         where 'me:'async_trait, T: std::marker::Sync {
+   |                               ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+