RFC 2203 not implemented inside a `const` block

[ecstatic-morse]

Thanks to RFC 2203, code like the following compiles on nightly:

fn main() {
    let _: [Option<Box<i32>>; 4] = [None; 4];
}

However, as pointed out by @RalfJung, the same code fails to compile in a const block (Playground):

const _: [Option<Box<i32>>; 4] = [None; 4];

Currently, there's two distinct implementations for promotion of temps, one for fn and const fn and one for const and static. RFC 2203 is implemented for the first but not the second.

I didn't see an actual issue for this. I'll close this as a dupe if @RalfJung already opened one.

cc #49147