Flesh out union case handling in IDL import:

umbrello/codeimport/idlimport.cpp
- In function parseStmt while-loop handling attribute declaration,
  if m_unionCases is non empty then
  - attempt finding the "idlCase" stereotype in UMLDoc;
  - if it is not found then create it and append the tag definition
    "label" of type String onto its attribute definitions;
  - set the stereotype on `attr';
  - append the joined m_unionCases labels onto attr->tags();
  - clear the content of m_unionCases.

CCBUG: 81301