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.

Co-authored-by: Jeff Bezanson <jeff.bezanson@gmail.com>