Fix #368

[MikeInnes]

The rough story here is that when a type like Complex{<:Real} gets written down in Julia code, it gets constructed dynamically by:

tv = Core.TypeVar(:s, Real)
body = Core.apply_type(Complex, tv)
Core.UnionAll(tv, body)

body is an intermediate state Complex{s<:Real} (notice no where s); it looks like a type but it isn't actually valid. However, generated functions, Core.Typeof etc still treat it as if it were a regular type, so it ends up inside the Pullback type, and this eventually breaks down.

julia> Core.Typeof(body)
Type{Complex{s<:Real}}

julia> Zygote.Pullback{Tuple{Core.Typeof(body)}}
Error showing value of type UnionAll:
ERROR: UndefVarError: S not defined

I'm not sure there's anything we can do about this in general; just @nograding some type functions is enough for now.