[clang] [NFC] [Serializer] Pack information in serializer (PR #69287)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@zygoloid @vgvassilev ping~

Personally I feel like the change itself should be safe. (The test for modules is really sensitive about the changes in Serializer)

https://github.com/llvm/llvm-project/pull/69287