Revert "[modules] Add experimental -fmodule-map-file-home-is-cwd flag to -cc1."

This reverts commit r223753.  It broke the Green Dragon build for a few
hours:

  http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-incremental_build/2259/
  http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-incremental_build/2259/consoleFull#43901905849ba4694-19c4-4d7e-bec5-911270d8a58c

I suspect `clang-tools-extra` just needs a follow-up for an API change,
but I'm not the right one to look into it.