Revert "[clang][dataflow] Correctly handle `InitListExpr` of union type." (#82856)

Reverts llvm/llvm-project#82348, which caused crashes when analyzing
empty InitListExprs for unions, e.g.

```cc
union U {
  double double_value;
  int int_value;
};

void target() {
  U value;
  value = {};
}
```

Co-authored-by: Samira Bazuzi <bazuzi@users.noreply.github.com>