Revert r310291, r310300 and r310332 because of test failure on Darwin

The commit r310291 introduced the failure. r310332 was a test fix commit and
r310300 was a followup commit. I reverted these two to avoid merge conflicts
when reverting.

The 'openmp-offload.c' test is failing on Darwin because the following
run lines:
// RUN:   touch %t1.o
// RUN:   touch %t2.o
// RUN:   %clang -### -no-canonical-prefixes -fopenmp=libomp -fopenmp-targets=nvptx64-nvidia-cuda -save-temps -no-canonical-prefixes %t1.o %t2.o 2>&1 \
// RUN:   | FileCheck -check-prefix=CHK-TWOCUBIN %s

trigger the following assertion:

Driver.cpp:3418:
    assert(CachedResults.find(ActionTC) != CachedResults.end() &&
           "Result does not exist??");

llvm-svn: 310345