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

[Timm Baeder via cfe-commits]

tbaederr wrote:

> 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.

Does this help at all? The patch is pretty obvious and I don't think it clears things up if I explain a single-line change a second time.

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