[clang] [Clang][ByteCode] Fix reading union template parameter object (PR #179899)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

I'm not sure if bit-fields should be handled specially. `visitInitList` handles bit-fields but the struct part of `visitAPValueInitializer` doesn't. I cannot find tests showing which is wrong so I don't add tests with bit-fields.

https://github.com/llvm/llvm-project/pull/179899