[clang] [C++20] [Modules] Introduce thin BMI (PR #71622)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Let's discuss the higher level problems in https://discourse.llvm.org/t/rfc-c-20-modules-introduce-thin-bmi-and-decls-hash/74755

https://github.com/llvm/llvm-project/pull/71622