[clang] [llvm] Added partial support for compiling C++20 modules and header-units without scanning. (PR #147682)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

LLVM itself can be built with clang header modules already. From compiler's perspective, header units is basically clang header modules. So if clang header modules can't improve LLVM 10x faster, I won't believe header units can.

https://github.com/llvm/llvm-project/pull/147682