Alloc hir::Lit in an arena to remove the destructor from Expr

This allows allocating `Expr`s into a dropless arena, which is useful
for using length prefixed thing slices in HIR, since these can only be
allocated in the dropless arena and not in a typed arena. This is
something I'm working on.

compiler/rustc_hir/src/arena.rs

@@ -51,6 +51,7 @@ macro_rules! arena_types {
             [] type_binding: rustc_hir::TypeBinding<'tcx>,
             [] variant: rustc_hir::Variant<'tcx>,
             [] where_predicate: rustc_hir::WherePredicate<'tcx>,
+            [] lit: rustc_hir::Lit,
         ]);
     )
 }

compiler/rustc_hir/src/hir.rs

@@ -1957,7 +1957,7 @@ pub enum ExprKind<'hir> {
     /// A unary operation (e.g., `!x`, `*x`).
     Unary(UnOp, &'hir Expr<'hir>),
     /// A literal (e.g., `1`, `"foo"`).
-    Lit(Lit),
+    Lit(&'hir Lit),
     /// A cast (e.g., `foo as f64`).
     Cast(&'hir Expr<'hir>, &'hir Ty<'hir>),
     /// A type reference (e.g., `Foo`).