Suggest to add missing feature when using gated const features

src/test/ui/error-codes/E0010-teach.stderr

@@ -12,6 +12,7 @@ error[E0019]: constant contains unimplemented expression type
 LL | const CON : Box<i32> = box 0;
    |                            ^
    |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
    = note: A function call isn't allowed in the const's initialization expression because the expression's value must be known at compile-time.
    = note: Remember: you can't use a function call inside a const's initialization expression! However, you can use it anywhere else.
 

src/test/ui/error-codes/E0010.stderr

@@ -9,6 +9,8 @@ error[E0019]: constant contains unimplemented expression type
    |
 LL | const CON : Box<i32> = box 0;
    |                            ^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 

src/test/ui/error-codes/E0017.stderr

@@ -12,6 +12,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0658]: references in statics may only refer to immutable values
   --> $DIR/E0017.rs:6:39

src/test/ui/error-codes/E0388.stderr

@@ -12,6 +12,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0658]: references in statics may only refer to immutable values
   --> $DIR/E0388.rs:5:39