[clang] [C++20] [Modules] Enable -fmodules-embed-all-files by default for named modules (PR #74419)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

I just found the implementation is not good since all the tests (they use %clang_cc1) won't use the new enabled feature but we decided to enable them all the time. So I'd like to find a better solution.

https://github.com/llvm/llvm-project/pull/74419