Remove C++-specific union codegen code.
Reuse getTypes in UnionConversions().

This should be equivalent, except that the removed code ignores return
values (which we currently don't support).
Merge UnionConversions() into UnionTypes().

It's not very useful to have one but not the other, so it makes more sense
to deal with them together.
auto merge of #2268 : Ms2ger/servo/unions, r=jdm