Stabilize `#![feature(const_if_match)]`
Quoting from the [stabilization report](https://github.com/rust-lang/rust/issues/49146#issuecomment-616301045):
> `if` and `match` expressions as well as the short-circuiting logic operators `&&` and `||` will become legal in all [const contexts](https://doc.rust-lang.org/reference/const_eval.html#const-context). A const context is any of the following:
>
> - The initializer of a `const`, `static`, `static mut` or enum discriminant.
> - The body of a `const fn`.
> - The value of a const generic (nightly only).
> - The length of an array type (`[u8; 3]`) or an array repeat expression (`[0u8; 3]`).
>
> Furthermore, the short-circuiting logic operators will no longer be lowered to their bitwise equivalents (`&` and `|` respectively) in `const` and `static` initializers (see #57175). As a result, `let` bindings can be used alongside short-circuiting logic in those initializers.
Resolves#49146.
Ideally, we would resolve 🐳#66753 before this lands on stable, so it might be worth pushing this back a release. Also, this means we should get the process started for #52000, otherwise people will have no recourse except recursion for iterative `const fn`.
r? @oli-obk
c977b8775d