[clang][DebugInfo] Respect fmodule-file-home-is-cwd in skeleton CUs for clang modules

When -fmodule-file-home-is-cwd and the path to the PCM is relative, we
shouldn't assume that the path to the PCM is relative to the modulemap
that produced it. To respect the option -fmodule-file-home-is-cwd, we
should assume the path is relative to the current working directory.

Reviewed By: rmaz

Differential Revision: https://reviews.llvm.org/D134911