Don't eval constants eagerly

[Nadrieril]

I made a mistake in my last PR: evaluating constants eagerly is semantically incorrect. For example, this is valid rust:

pub trait HasLen {
    const LEN: usize;
}

impl<const N: usize> HasLen for [bool; N] {
    const LEN: usize = if true {
        N
    } else {
        <Self as HasLen<N>>::LEN
    };
}

but eagerly evaluating causes cycle errors. Promoted constants however should be evaluatable since they're only promoted when they're infallible.