thir: Add Become expression kind

compiler/rustc_middle/src/thir.rs

@@ -410,6 +410,10 @@ pub enum ExprKind<'tcx> {
     Return {
         value: Option<ExprId>,
     },
+    /// A `become` expression.
+    Become {
+        value: ExprId,
+    },
     /// An inline `const` block, e.g. `const {}`.
     ConstBlock {
         did: DefId,

compiler/rustc_middle/src/thir/visit.rs

@@ -100,6 +100,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
                 visitor.visit_expr(&visitor.thir()[value])
             }
         }
+        Become { value } => visitor.visit_expr(&visitor.thir()[value]),
         ConstBlock { did: _, substs: _ } => {}
         Repeat { value, count: _ } => {
             visitor.visit_expr(&visitor.thir()[value]);