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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@mizvekov would you like to take a look at this? This is related (in some level) to what you say https://github.com/llvm/llvm-project/pull/79959. In short, in the direction, in the one-phase compilation, the pcm won't be compiled into object files. (But it still will in the two-phase compilation).

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