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

[Michael Park via cfe-commits]

mpark wrote:

Separately, even with named modules, with or without `-fmodules-embed-all-files`, the `INPUT_FILE` paths in the PCM are still absolute. I guess with `-fmodules-embed-all-files` those paths are not used since the source files are embedded, which I suppose is how it avoids problems with the absolute paths. But if we want relocatable modules without using `-fmodules-embed-all-files` though, is there a solution for that?

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