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

[Paul Schwabauer via cfe-commits]

koplas wrote:

Do I miss something? The performance and file size is similar with and without `-fgen-reduced-bmi`.
To reproduce clone https://github.com/koplas/clang-modules-test and run `build.sh` and `build_thin_bmi.sh`.

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