Stabilize the compile_error_macro feature

Stabilizes:

* `compile_error!` as a macro defined by rustc

Closes #40872

src/test/compile-fail/compile_error_macro.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(compile_error)]
-
 fn main() {
     compile_error!("a very descriptive error message"); //~ ERROR: a very descriptive error message
 }

src/test/compile-fail/feature-gate-compile_error.rs

@@ -1,13 +0,0 @@
-// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-fn main() {
-    compile_error!("test"); //~ ERROR: `compile_error` is not stable enough
-}