Do not fold IsConst applied to dependent parameters

[mbovel]

IsConst is currently folded too early when applied to dependent parameters, before they have a chance to be substituted. Here is an example of the problem:

val x: 2 = 2

summon[IsConst[x.type] =:= true] // works

def isConst(arg: Int)(using IsConst[arg.type] =:= true): Unit = ???
isConst(x) // error; IsConst[(arg: Int)] =:= false

This PR fixes this issue, so that the second case also works.

[mbovel]

Why did you merge this PR @Kordyjan? It might have ended it the list of back-ported PRs by mistake.

I think it's ok to have it merged, but it would still be nice if someone could review it.

[Kordyjan]

That is something peculiar. I have no clue why this PR landed on the backports list. I'll try to get to know what happened. I also need to check if that means some legit backport for 3.2 was not merged.

I'm sorry for the situation.