[clang] [C++20] [Modules] Bring Decls Hash to BMI for C++20 Module units (PR #71627)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

As I summarized in https://discourse.llvm.org/t/rfc-c-20-modules-introduce-thin-bmi-and-decls-hash/74755/43, people are interested in this direction, but it may be too early for us to implement it. Let's postpone this.

https://github.com/llvm/llvm-project/pull/71627