[clang] Support '-fmodule-file-home-is-cwd' for C++ modules. (PR #135147)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

Yeah, I have some concern about the interface. But I am in the camp that we'd better to get best practice from practice instead of pure imagination. And given Meta is doing the experiments in practice, I think it is fine to let it go.

https://github.com/llvm/llvm-project/pull/135147