Stabilize const_black_box

This has been unstably const since [1], but a tracking issue was never
created. Per discussion on Zulip [2], there should not be any blockers
to making this const-stable. The function does not provide any
functionality at compile time but does allow code reuse between const-
and non-const functions, so stabilize it here.

[1]: rust-lang#92226
[2]: https://rust-lang.zulipchat.com/#narrow/channel/146212-t-compiler.2Fconst-eval/topic/const_black_box

Author: tgross35

tests/run/int.rs

@@ -3,8 +3,6 @@
 // Run-time:
 //   status: 0
 
-#![feature(const_black_box)]
-
 /*
  * Code
  */