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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

My impression to the feedbacks is that every one of us loves the direction, while we may need more agreement on the user interfaces.

To make it easier to review, I split all the user interfaces related part to following patches. So that the current patch won't affect users.

This patch introduced an CC1 option `-emit-thin-module-interface` and tests the behavior for almost every places we can to make sure it is stable. While the diff is big, we can find most parts of it are boiler plate, so don't be panic.

I feel the patch good and hope we can land this quickly so that we can start the work to help users actually.



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