Revert r310489 and follow-up commits r310505, r310519, r310537 and r310549

Commit r310489 caused 'openmp-offload.c' test failures on Darwin and other
platforms:
http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-incremental_check/39230/testReport/junit/Clang/Driver/openmp_offload_c/

The follow-up commits tried to fix the test, but the test is still failing.

llvm-svn: 310580