In IDLWriter flesh out code generation for IDL union:

umbrello/codegenerators/idl/idlwriter.cpp
- In function writeClass, if `stype' contains "Union" then
  - retrieve the classifier's attributes into local `atl';
  - pop first attribute off front of `atl' into local `discrim' and
    generate the "switch" type using discrim;
  - iterate over the remaining elements of `atl' and handle tags on
    each element (the tags are used for storing the union case labels
    or the "default" label) before printing each attribute.
- Additional change in function writeClass: On handling Typedef fix
  crash when superclass is not set.

CCBUG: 81301