[clang] [clang] Fix assertion failure in constexpr union deserialization (PR #140179)

[Matheus Izvekov via cfe-commits]

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

LGTM

Something like:
* Fixes serialization of constexpr structs containing unions. (#GH140130)

https://github.com/llvm/llvm-project/pull/140179