Don't reuse bindings for ref mut

Reusing bindings causes errors later in lowering:

```
 error[E0596]: cannot borrow `vec` as mutable, as it is not declared as mutable
  --> /checkout/src/test/ui/async-await/argument-patterns.rs:12:20
   |
LL | async fn b(n: u32, ref mut vec: A) {
   |                    ^^^^^^^^^^^
   |                    |
   |                    cannot borrow as mutable
   |                    help: consider changing this to be mutable: `mut vec`
```

compiler/rustc_ast_lowering/src/item.rs

@@ -1102,6 +1102,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         ident,
                         _,
                     ) => (ident, true),
+                    // For `ref mut` arguments, we can't reuse the binding, but
+                    // we can keep the same name for the parameter.
+                    // This lets rustdoc render it correctly in documentation.
+                    hir::PatKind::Binding(_, _, ident, _) => (ident, false),
                     _ => {
                         // Replace the ident for bindings that aren't simple.
                         let name = format!("__arg{}", index);