Use Lit rather than P<Lit> in hir::ExprKind.

It's simpler and makes some benchmark run up to 1% faster. It also makes
`hir::ExprKind` more like `ast::ExprKind` (which underwent the
equivalent change in #55777).

src/librustc/hir/lowering.rs

@@ -3775,7 +3775,7 @@ impl<'a> LoweringContext<'a> {
                 let ohs = P(self.lower_expr(ohs));
                 hir::ExprKind::Unary(op, ohs)
             }
-            ExprKind::Lit(ref l) => hir::ExprKind::Lit(P((*l).clone())),
+            ExprKind::Lit(ref l) => hir::ExprKind::Lit((*l).clone()),
             ExprKind::Cast(ref expr, ref ty) => {
                 let expr = P(self.lower_expr(expr));
                 hir::ExprKind::Cast(expr, self.lower_ty(ty, ImplTraitContext::disallowed()))

src/librustc/hir/mod.rs

@@ -1466,7 +1466,7 @@ pub enum ExprKind {
     /// A unary operation (For example: `!x`, `*x`)
     Unary(UnOp, P<Expr>),
     /// A literal (For example: `1`, `"foo"`)
-    Lit(P<Lit>),
+    Lit(Lit),
     /// A cast (`foo as f64`)
     Cast(P<Expr>, P<Ty>),
     Type(P<Expr>, P<Ty>),