[clang] [C++20] [Modules] Introduce reduced BMI (PR #75894)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

I am going to land this in the week later if no objections come in. I think it is necessary to land the series of patches (to reduce the contents of BMI) for clang19. And of course, the functionality will be opt in for one~two releases for experimental.

https://github.com/llvm/llvm-project/pull/75894