CodeGen v1 does not record anything for pointers to incomplete types.
Not even the address, as is done for other pointers.
Introduce a new Primitive type "Incomplete". This behaves identically to
"Void", but allows us to tell whether a type was defined as void or if
it ended up like that because of incomplete DWARF information.
This extracts the compatibility logic from AddPadding, which allows for it to be
simplified and will make it easier to extend and eventually remove in the
future. No functional changes.
