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

[Iain Sandoe via cfe-commits]

iains 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)


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