Add feature-gates for desugaring-based box and placement-in.

update test/compile-fail/feature-gate-box-expr.rs to reflect new feature gates.

Part of what lands with Issue 22181.

src/libstd/lib.rs

@@ -234,6 +234,7 @@
 #![feature(no_std)]
 #![feature(oom)]
 #![feature(optin_builtin_traits)]
+#![feature(placement_in_syntax)]
 #![feature(rand)]
 #![feature(raw)]
 #![feature(reflect_marker)]