const fn: Missing tracking issue and feature gate name in stability error messages

[SimonSapin]

In rustc 1.33.0-nightly (a2b0f24 2018-12-30), compare for example:

error: function pointers in const fn are unstable
 --> a.rs:1:26
  |
1 | const fn foo(x: fn()) -> fn() { x }
  |                          ^^^^

error: aborting due to previous error

… with the error message for some other unstable language feature:

error[E0658]: box expression syntax is experimental; you can call `Box::new` instead. (see issue #49733)
 --> a.rs:1:24
  |
1 | fn foo() -> Box<u32> { box 4 }
  |                        ^^^^^
  |
  = help: add #![feature(box_syntax)] to the crate attributes to enable

error: aborting due to previous error

[kadiwa4]

const_fn_fn_ptr_basics has been stabilized in #93827 and no longer emits an error.