constructing tuples: some reasonable `Union` tuple type constructors throw

[nsajko]

julia> Tuple{Int,Int}(Any[1.0,2.0])
(1, 2)

julia> Union{Tuple{Int,Int},Tuple{Float64}}(Any[1.0,2.0])
ERROR: MethodError: no method matching argtail()

Closest candidates are:
  argtail(::Any, ::Any...)
   @ Base essentials.jl:466

Stacktrace:
 [1] tuple_type_tail(T::Type)
   @ Base ./tuple.jl:446
 [2] tuple_type_tail(T::Type)
   @ Base ./tuple.jl:438
 [3] _totuple
   @ ./tuple.jl:472 [inlined]
 [4] _totuple
   @ ./tuple.jl:473 [inlined]
 [5] Union{Tuple{Float64}, Tuple{Int64, Int64}}(itr::Vector{Any})
   @ Base ./tuple.jl:455
 [6] top-level scope
   @ REPL[4]:1

julia> versioninfo()
Julia Version 1.11.0-DEV.1451
Commit f117a500ca9 (2024-02-01 15:38 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver2)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)

IMO we should instead have (1, 2) === Union{Tuple{Int,Int},Tuple{Float64}}(Any[1.0,2.0]). Working on a PR that should fix this, and other, issues.