hir: Add Become expression kind

compiler/rustc_hir/src/intravisit.rs

@@ -791,6 +791,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
         ExprKind::Ret(ref optional_expression) => {
             walk_list!(visitor, visit_expr, optional_expression);
         }
+        ExprKind::Become(ref expr) => visitor.visit_expr(expr),
         ExprKind::InlineAsm(ref asm) => {
             visitor.visit_inline_asm(asm, expression.hir_id);
         }