min_const_generics: allow ty param in repeat expr

[lcnr]

implements https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/repeat.20expressions

Even with min_const_generics active, now keeps resulting in future compat warnings instead of hard errors.
Const parameters, for example [0; N + 1], still result in hard errors during resolve.

#![allow(dead_code)]

fn foo<T>() {
    [0; std::mem::size_of::<*mut T>()];
}

struct Foo<T>(T);

impl<T> Foo<T> {
    const ASSOC: usize = 4;

    fn test() {
        [0; Self::ASSOC];
    }
}

r? @varkor cc @petrochenkov

[varkor]

@bors r+ rollup

[bors]

📌 Commit 83ecbb4 has been approved by varkor