Implement TC's match ergonomics 2024 proposal

Under gate `ref_pat_eat_one_layer_2024_structural`.
Enabling `ref_pat_eat_one_layer_2024` at the same time allows the union
of what the individual gates allow.

compiler/rustc_ast/src/ast.rs

@@ -711,6 +711,7 @@ pub enum ByRef {
 }
 
 impl ByRef {
+    #[must_use]
     pub fn cap_ref_mutability(mut self, mutbl: Mutability) -> Self {
         if let ByRef::Yes(old_mutbl) = &mut self {
             *old_mutbl = cmp::min(*old_mutbl, mutbl);