[clang] [C++20] [Modules] Introduce thin BMI (PR #71622)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

The term `sparse` makes me wondering its format is not efficiency.. And for the term `surface`, ..., it may beyond my knowledge,  I feel it is odd but I can't give a concrete reason.

https://github.com/llvm/llvm-project/pull/71622