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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Oh, I didn't notice libcxx will test with clang17 and clang18 too. Where we didn't introduce reduced bmi.  We can't use `-Wno-missing-reduced-bmi` since it won't be recognized by clang17 and clang18.

@ldionne @mordante do you think how can we proceed on this?

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