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

[Michael Park via cfe-commits]

mpark wrote:

Hm.. I'm not too sure about C++ named modules but for header units, the absolute path of an imported PCM via `-fmodule-file=` gets included in the produced PCM. I think that the `-fmodule-file-home-is-cwd` flag (already existing, I'm not proposing to add a new one) should be respected to produce paths relative to CWD of the imported PCMs.

As far as I can tell, `-fmodules-embed-all-files` seems to be about embedding source files into the PCMs or something, which doesn't deal with imported PCM paths. Is that correct?

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