[clang] [[CIR]] Implement 'to-union' cast. (PR #191485)

[Bruno Cardoso Lopes via cfe-commits]

https://github.com/bcardosolopes approved this pull request.

LGTM. Minor nit: existing test is `cast.cpp` (singular) — might be worth matching as `cast.c`   
  for consistency, but not a blocker.

https://github.com/llvm/llvm-project/pull/191485