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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> Do you expect to make any changes to type streaming?

I don't expect to do that explicitly. The number of types deserialized can be decreased naturally after we avoid emitting declarations during the writing.

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