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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Got it. I've renamed the flag as `-fexperimental-modules-reduced-bmi`. I feel the suggestion like let users to use `-Xclang` options look odd..

---

> What do you mean by "symmetric"?

I mean, we always want the users to do something explicitly to enable the feature. But this might not be important now.


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