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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Since there is no meaningful review opinions here, let's continue it in a new and clean PR: https://github.com/llvm/llvm-project/pull/75894

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