Use explicit promotion for constants in repeat expressions

[oli-obk]

As per #49147 (comment)

Even in generics everything is obvious. Either you have a Copy bound, or you get promotion. I think not being a full const context will confuse more than it will not. We'll just get requests to add more things or at least issues opened asking why things that are obv OK don't work

cc @rust-lang/wg-const-eval