[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`.
> 
> Thanks.
> 
> > I feel the suggestion like let users to use `-Xclang` options look odd..
> 
> I think the point here is that we want **expert** users to try this out (with understanding that it might not behave exactly as they expect). Because it is experimental, it is not yet ready for "end users" - I would expect expert users to be able to deal with -Xclang xxxxx (but I will not insist, it's just a suggestion)

Agreed.

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