[clang] [clang][bytecode][NFC] Add missing LLVM_PREFERRED_TYPE to bitfield (PR #128740)

[Shafik Yaghmour via cfe-commits]

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

LGTM but please add a more detailed summary something along the lines of `Added LLVM_PREFERRED_TYPE to InUnion field of InlineDescriptor` would be sufficient.

https://github.com/llvm/llvm-project/pull/128740