Fix fieldtype_tfunc for union types

[martinholters]

On master:

julia> T = Union{Tuple{Val}, Tuple{Tuple}}
Union{Tuple{Val}, Tuple{Tuple}}

julia> fieldtype(T, 1)
Union{Tuple, Val}

julia> Core.Compiler.fieldtype_tfunc(Core.Const(T), Core.Const(1))
Union{Type{Val}, Type{Tuple}} # should be Type{Union{Tuple, Val}}

I tried to be conservative here in the sense that in cases where this PR changes the result, it is actually for the better, but there may be cases left where fieldtype_tfunc remains unsound for Type{Union{...}} input.

Fixes #40177.

[martinholters]

I think the CI failure on FreeBSD is unrelated/also seen elsewhere? Then this should be good to go.