This not only reduces code duplication, but the unification
also enables code a la StructLiteralExp to handle classes
(for CTFE class constant support in 2.063).
This is just to improve clarity, as it was rather non-obvious
what of the code also applied to classes before.
IrTypeAggr::createInitializerConstant would currently belong in
IrTypeStruct, but this will be refactored anyway.