Normalize (simplify) UnionAlls when used as type parameter

[martinholters]

A stab at #35130 although I'm not totally convinced this is a good idea, see #35130 (comment).

This normalizes UnionAlls (and Unions thereof) when they are used as type parameters. Normalization here means that the typevar is replaced with its upper bound if it does not appear in invariant position and is not subject to the diagonal rule (or in the trivial case where lower and upper bound are identical). Parameters to Type are excluded from this normalization to prevent the change in dispatch also mentioned in #35130 (comment). This is sufficient to cover the cases from #35130:

julia> Val{Tuple{T} where T}
Val{Tuple{Any}}

julia> Val{Tuple{Float64,Vararg{T,2}} where T<:Int64}
Val{Tuple{Float64,Int64,Int64}}

julia> Val{Tuple{Float64,T,T} where T<:Int64}
Val{Tuple{Float64,Int64,Int64}}

julia> Val{Union{Nothing, T} where T}
Val{Any}

This mostly also makes the tests from #34272 pass (except for where the Type exclusion is relevant), but I'm not convinced there aren't other cases where type-equal concrete types are not normalized to identical types, so #34272 might still be necessary in all its glory.

Closes #35130.

[JeffBezanson]

I updated and rebased this. I think we should probably merge it, since it fixes some crashes. Seems to be a strict improvement.

[KristofferC]

@JeffBezanson, this needs a manual backport to #40209

[Sacha0]

Thanks for shepherding this along Jeff, and for having an eye on the backport Kristoffer! :)