On resolve error of [rest..], suggest [rest @ ..]

When writing a pattern to collect multiple entries of a slice in a
single binding, it is easy to misremember or typo the appropriate syntax
to do so, instead writing the experimental `X..` pattern syntax. When we
encounter a resolve error because `X` isn't available, we suggest
`X @ ..` as an alternative.

```
error[E0425]: cannot find value `rest` in this scope
  --> $DIR/range-pattern-meant-to-be-slice-rest-pattern.rs:3:13
   |
LL |         [1, rest..] => println!("{rest:?}"),
   |             ^^^^ not found in this scope
   |
help: if you meant to collect the rest of the slice in `rest`, use the at operator
   |
LL |         [1, rest @ ..] => println!("{rest:?}"),
   |                  +
```

Fix #88404.

compiler/rustc_resolve/src/late.rs

@@ -603,6 +603,8 @@ struct DiagnosticMetadata<'ast> {
     /// Only used for better errors on `let <pat>: <expr, not type>;`.
     current_let_binding: Option<(Span, Option<Span>, Option<Span>)>,
 
+    current_pat: Option<&'ast Pat>,
+
     /// Used to detect possible `if let` written without `let` and to provide structured suggestion.
     in_if_condition: Option<&'ast Expr>,
 
@@ -703,6 +705,12 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
     fn visit_expr(&mut self, expr: &'ast Expr) {
         self.resolve_expr(expr, None);
     }
+    fn visit_pat(&mut self, p: &'ast Pat) {
+        let prev = self.diagnostic_metadata.current_pat;
+        self.diagnostic_metadata.current_pat = Some(p);
+        visit::walk_pat(self, p);
+        self.diagnostic_metadata.current_pat = prev;
+    }
     fn visit_local(&mut self, local: &'ast Local) {
         let local_spans = match local.pat.kind {
             // We check for this to avoid tuple struct fields.