tuple_tfunc bug handling of Union containing Type{...}

[aviatesk]

I guess this issue should be raised before, but I couldn't find an issue that discusses this.
When working on #44512 (in particularity this issue), I found the following isa check doesn't work as I expected:

julia> Type{Int} <: DataType
true

julia> Int isa Type{Int}
true

julia> (Int,) isa Tuple{DataType} # expected
true

julia> (Int,) isa Tuple{Type{Int}} # I expected this to return `true`
false

So in essence Tuple-type whose parameter contains Type-type doesn't seem to follow the covariant rule.

This issue leads to the ERROR: fatal error in type inference (type bound) error in #44512 when we try to union-split Base.indexed_iterate(::Union{Tuple{Nothing, Nothing, Nothing}, Tuple{Any, Union{Type{AbstractContinuousSystem}, Type{AbstractDiscreteSystem}}, Any}}, ...) callsite.

[vtjnash]

That is an odd looking signature type, since we should be unable to ever select that part of the union. There do not exist any values which satisfy isa Tuple{Type{Int}}, since Type{Int} is a subtype of DataType, which is the concrete type of this value. In particular, and for Tuples only, we can have subtypes of concrete types (but not concrete subtypes of concrete subtypes), which can lead to some confusion over the dispatch behavior if one is not careful about reflecting this fact in the algorithms.

[vtjnash]

incorrect:

julia> Core.Compiler.tuple_tfunc(Any[Union{Type{Int32}, Type{Int64}}])
Tuple{Union{Type{Int32}, Type{Int64}}}

correct:

julia> Core.Compiler.tmerge(Core.Compiler.tuple_tfunc(Any[Type{Int32}]), Core.Compiler.tuple_tfunc(Any[Type{Int64}]))
Tuple{DataType}