lowering: Disallow splatting in non-final default value

[Keno]

Pop quiz: Do you know what the following will do?

julia> function g1(a=(1,2)..., b...=3)
    b
end

julia> g1()

julia> function g2(a=(1,2)..., b=3, c=4)
    (b, c)
end

julia> g2()

julia> function g3(a=(1,2)..., b=3, c...=4)
    (b, c)
end

julia> g3()

julia> g3(1)

I don't either and I don't think it's particularly well defined. Splatting a default argument makes sense on the last argument, which can be a vararg, and it is desirable to be able to specify the default for the whole varargs tuple at once (although arguably that should just be the non-... behavior, but that'd be too breaking a change). Ref #50518. However, for other arguments, there isn't really a sensible semantic meaning. This PR disallows this in lowering. This is technically a minor change, but I doubt anybody is using this. Splatting in default values wasn't really ever supposed to work anyway, it just happened to fall out of our lowering.

[ericphanson]

Is the variant from the issue still allowed?

function f(xs...=["a", "b", "c"]...; debug=false)
           return xs[1]
end

Maybe worth a test?

[Keno]

Yes, it's allowed and was added as a test in #50559.

[maleadt]

This is technically a minor change, but I doubt anybody is using this.

Let's see if Nanosoldier finds any packages:

@nanosoldier runtests()

[nanosoldier]

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