[clang] [C++20] [Modules] Introduce reduced BMI (PR #75894)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@iains would you like to take a look on this. I feel OK to not land this in 18 but I think it is really important to land this in 19. Although it is still early to that, there are other works to do (e.g., the next patch and more testing). So it may be better to start it earlier.

https://github.com/llvm/llvm-project/pull/75894