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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> Going back to the idea of not writing the paths of imported PCMs at all; currently the condition to not write the paths is if the imported PCM is a named module. Perhaps this condition can be extended to omit the paths if the PCM was found through `-fmodule-file=<name>=<pcm>`?

IIRC, this relates to build systems. While not writing the imported BMI paths in the BMI, the cost is, the build system must provide a full set of `-fmodule-file=<name>=<pcm>` for all directly or indirectly imported modules. This is a convention between named modules and build systems (or cmake, in particular).

I think this is not related to `-fmodule-file=<name>=<pcm>` or not. In this way, we need to build a new convention for header units. 

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