Point `placement_in_syntax`/`box_syntax` lang features at tracking issue #27779.

cc #28244.

src/libsyntax/feature_gate.rs

@@ -81,8 +81,8 @@
     ("associated_types", "1.0.0", None, Accepted),
     ("visible_private_types", "1.0.0", None, Active),
     ("slicing_syntax", "1.0.0", None, Accepted),
-    ("box_syntax", "1.0.0", None, Active),
-    ("placement_in_syntax", "1.0.0", None, Active),
+    ("box_syntax", "1.0.0", Some(27779), Active),
+    ("placement_in_syntax", "1.0.0", Some(27779), Active),
     ("pushpop_unsafe", "1.2.0", None, Active),
     ("on_unimplemented", "1.0.0", None, Active),
     ("simd_ffi", "1.0.0", None, Active),