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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

It looks the CI will compile libc++ with "old" clang19 and the just built clang. So it might not be able to recognize the new flag. Tried to use `-fexperimental-modules-reduced-bmi` for libc++. We can replace it with `-fmodules-reduced-bmi` someday after we don't compile it with clang19 and previous version.

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