[clang] [C++20] [Modules] Introduce -fgen-reduced-bmi (PR #85050)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@iains @dwblaikie Understood. And I thought the major problem is that there are a lot flags from clang modules. And it is indeed confusing. But given we have to introduce new flags in this case, probably we can only try to make it more clear by better documents.

https://github.com/llvm/llvm-project/pull/85050