[clang] [C++20] [Modules] Convert '-fexperimental-modules-reduced-bmi' to '-fmodules-reduced-bmi' (PR #114382)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> I think the overall plan is fine, thanks for working on it!
> 
> I think the idea of issuing a warning in order to get users to opt-in to a flag is somewhat novel. Have I missed discussion about this approach, versus simply changing the default?

I originally mentioned it in https://github.com/llvm/llvm-project/pull/85050 and then in the document. And it looks like no one had a question about it.

> 
> Regardless, for any new warning we add here, we will probably need to add a targeted way to disable that warning, as otherwise users of Werror are likely to complain.

We can disable it with the same way with other warnings, e.g., `-Wno-missing-reduced-bmi`



https://github.com/llvm/llvm-project/pull/114382